Fundamental Trigonometric Identities

Trigonometric identities are mathematical relationships that connect different trigonometric functions. They allow us to rewrite complicated expressions, establish equivalent relationships, and solve problems involving angles, triangles, waves, and periodic motion.

A trigonometric identity is an equation that is true for every value of the angle for which both sides are defined.

For example:

sin² θ + cos² θ = 1

This relationship is true for every real angle θ.

The fundamental trigonometric identities can be organised into three main groups:

• Reciprocal identities, which connect trigonometric functions with their reciprocals.

• Quotient identities, which express tangent and cotangent in terms of sine and cosine.

• Pythagorean identities, which are derived from the Pythagorean theorem and the unit circle.

Rather than simply memorising these identities, understanding how they are derived makes it easier to recognise when and how to use them.

Foundations: The Six Trigonometric Functions

The six trigonometric functions can be defined using a right triangle.

For an acute angle θ, the three sides are:

• Opposite: The side across from θ.

• Adjacent: The side next to θ, excluding the hypotenuse.

• Hypotenuse: The longest side, opposite the right angle.

The three primary trigonometric ratios are:

sin θ = opposite/hypotenuse

cos θ = adjacent/hypotenuse

tan θ = opposite/adjacent

The other three functions are:

csc θ = hypotenuse/opposite

sec θ = hypotenuse/adjacent

cot θ = adjacent/opposite

These six definitions provide the foundation for deriving the reciprocal and quotient identities.

Although the triangle definitions begin with acute angles, the identities extend to other angles through the unit circle.

Deriving the Reciprocal Identities

A reciprocal is obtained by exchanging the numerator and denominator of a nonzero fraction.

For example, the reciprocal of 3/5 is 5/3.

Two nonzero numbers that are reciprocals have a product of 1.

Deriving the Cosecant Identity

Start with the definition of sine:

sin θ = opposite/hypotenuse

Take the reciprocal:

1/sin θ = hypotenuse/opposite

But:

csc θ = hypotenuse/opposite

Therefore:

csc θ = 1/sin θ

Equivalently:

sin θ = 1/csc θ

We can also multiply both functions:

sin θ × csc θ = 1

This relationship is valid wherever sin θ ≠ 0.

Deriving the Secant Identity

Start with:

cos θ = adjacent/hypotenuse

Take the reciprocal:

1/cos θ = hypotenuse/adjacent

Since:

sec θ = hypotenuse/adjacent

we obtain:

sec θ = 1/cos θ

Equivalently:

cos θ = 1/sec θ

Therefore:

cos θ × sec θ = 1

This relationship is valid wherever cos θ ≠ 0.

Deriving the Cotangent Identity

Start with:

tan θ = opposite/adjacent

Take the reciprocal:

1/tan θ = adjacent/opposite

Since:

cot θ = adjacent/opposite

we obtain:

cot θ = 1/tan θ

Equivalently:

tan θ = 1/cot θ

Therefore:

tan θ × cot θ = 1

The reciprocal relationship is valid where both tangent and cotangent are defined, requiring sin θ ≠ 0 and cos θ ≠ 0.

Reciprocal Identity Summary

Worked Example

Suppose:

sin θ = 5/13

Find csc θ.

Using:

csc θ = 1/sin θ

Substitute:

csc θ = 1/(5/13)

Therefore:

csc θ = 13/5

Deriving the Quotient Identities

A quotient is the result of dividing one quantity by another.

The quotient identities express tangent and cotangent in terms of sine and cosine.

Deriving the Tangent Quotient Identity

Start with:

sin θ = opposite/hypotenuse

cos θ = adjacent/hypotenuse

Divide sine by cosine:

sin θ/cos θ

= (opposite/hypotenuse)/(adjacent/hypotenuse)

Multiply by the reciprocal of the denominator:

= (opposite/hypotenuse) × (hypotenuse/adjacent)

Cancel the hypotenuse:

= opposite/adjacent

But:

tan θ = opposite/adjacent

Therefore:

tan θ = sin θ/cos θ

This identity is valid wherever cos θ ≠ 0.

Deriving the Cotangent Quotient Identity

Start with:

cos θ = adjacent/hypotenuse

sin θ = opposite/hypotenuse

Divide cosine by sine:

cos θ/sin θ

= (adjacent/hypotenuse)/(opposite/hypotenuse)

Multiply by the reciprocal:

= (adjacent/hypotenuse) × (hypotenuse/opposite)

Cancel the hypotenuse:

= adjacent/opposite

But:

cot θ = adjacent/opposite

Therefore:

cot θ = cos θ/sin θ

This identity is valid wherever sin θ ≠ 0.

Quotient Identity Summary

These identities are especially useful because they allow expressions containing several different trigonometric functions to be rewritten using only sine and cosine.

Deriving Quotient Identities Using the Unit Circle

The unit circle provides another way to understand the quotient identities.

For an angle θ, the corresponding point on the unit circle has coordinates:

(cos θ, sin θ)

Therefore:

x = cos θ

y = sin θ

The tangent ratio is:

tan θ = y/x

Substituting the coordinates:

tan θ = sin θ/cos θ

Similarly:

cot θ = x/y

Therefore:

cot θ = cos θ/sin θ

This derivation shows that quotient identities are not limited to acute angles in right triangles.

They also apply to angles in other quadrants, provided the denominators are nonzero.

Deriving the Fundamental Pythagorean Identity

The most important trigonometric identity is:

sin² θ + cos² θ = 1

This relationship comes directly from the Pythagorean theorem.

For a right triangle with side lengths a, b, and hypotenuse c:

a² + b² = c²

Suppose:

a = opposite

b = adjacent

c = hypotenuse

Then:

opposite² + adjacent² = hypotenuse²

Divide every term by hypotenuse²:

opposite²/hypotenuse² + adjacent²/hypotenuse² = 1

Rewrite each fraction:

(opposite/hypotenuse)² + (adjacent/hypotenuse)² = 1

Using the definitions of sine and cosine:

sin² θ + cos² θ = 1

Therefore:

sin² θ + cos² θ = 1

Why This Identity Is Fundamental

This identity connects sine and cosine directly.

It allows us to calculate one function when the other is known.

It also forms the basis for deriving the other two Pythagorean identities.

Deriving the Pythagorean Identity from the Unit Circle

The equation of a circle centred at the origin with radius r is:

x² + y² = r²

For the unit circle:

r = 1

Therefore:

x² + y² = 1

A point on the unit circle has coordinates:

x = cos θ

y = sin θ

Substitute:

cos² θ + sin² θ = 1

Therefore:

sin² θ + cos² θ = 1

This geometric argument establishes the identity for every real angle.

Deriving the Second Pythagorean Identity

Start with:

sin² θ + cos² θ = 1

Divide every term by cos² θ:

sin² θ/cos² θ + cos² θ/cos² θ = 1/cos² θ

Simplify:

sin² θ/cos² θ = tan² θ

cos² θ/cos² θ = 1

1/cos² θ = sec² θ

Therefore:

tan² θ + 1 = sec² θ

Usually written:

1 + tan² θ = sec² θ

This identity is valid wherever cos θ ≠ 0.

Rearranging the Identity

Subtract 1:

tan² θ = sec² θ − 1

Subtract tan² θ from both sides:

1 = sec² θ − tan² θ

All three forms are useful when simplifying expressions.

Deriving the Third Pythagorean Identity

Again, start with:

sin² θ + cos² θ = 1

Divide every term by sin² θ:

sin² θ/sin² θ + cos² θ/sin² θ = 1/sin² θ

Simplify:

sin² θ/sin² θ = 1

cos² θ/sin² θ = cot² θ

1/sin² θ = csc² θ

Therefore:

1 + cot² θ = csc² θ

This identity is valid wherever sin θ ≠ 0.

Rearranging the Identity

Subtract 1:

cot² θ = csc² θ − 1

Subtract cot² θ:

1 = csc² θ − cot² θ

Pythagorean Identity Summary

Using Identities to Calculate Unknown Values

Trigonometric identities allow us to determine unknown function values without calculating the angle itself.

Worked Example

Suppose:

sin θ = 3/5

and θ lies in Quadrant II.

Find cos θ, tan θ, and sec θ.

Find Cosine

Use:

sin² θ + cos² θ = 1

Substitute:

(3/5)² + cos² θ = 1

9/25 + cos² θ = 1

Subtract:

cos² θ = 16/25

Take the square root:

cos θ = ±4/5

Since θ lies in Quadrant II, cosine is negative.

Therefore:

cos θ = −4/5

Find Tangent

Use the quotient identity:

tan θ = sin θ/cos θ

= (3/5)/(−4/5)

Therefore:

tan θ = −3/4

Find Secant

Use the reciprocal identity:

sec θ = 1/cos θ

= 1/(−4/5)

Therefore:

sec θ = −5/4

Why the Quadrant Matters

When solving an equation such as:

cos² θ = 16/25

both positive and negative square roots are mathematically possible.

The quadrant determines the correct sign.

Simplifying Expressions Using Reciprocal Identities

Reciprocal identities often simplify products involving two related trigonometric functions.

Worked Example

Simplify:

sin θ csc θ

Replace cosecant:

sin θ(1/sin θ)

Cancel the common factor:

= 1

Valid where sin θ ≠ 0.

Worked Example

Simplify:

cos θ sec θ + sin θ csc θ

Apply the reciprocal identities:

= cos θ(1/cos θ) + sin θ(1/sin θ)

Simplify each product:

= 1 + 1

Therefore:

= 2

This is valid where both sine and cosine are nonzero.

Simplifying Expressions Using Quotient Identities

Worked Example

Simplify:

tan θ cos θ

Use:

tan θ = sin θ/cos θ

Substitute:

= (sin θ/cos θ)cos θ

Cancel cosine:

= sin θ

Valid where cos θ ≠ 0.

Worked Example

Simplify:

cot θ sin θ

Use:

cot θ = cos θ/sin θ

Substitute:

= (cos θ/sin θ)sin θ

Simplify:

= cos θ

Valid where sin θ ≠ 0.

Worked Example

Simplify:

tan θ/sec θ

Replace both functions:

= (sin θ/cos θ)/(1/cos θ)

Multiply by the reciprocal:

= (sin θ/cos θ)cos θ

Therefore:

= sin θ

Valid where cos θ ≠ 0.

Simplifying Expressions Using Pythagorean Identities

Worked Example

Simplify:

1 − cos² θ

Use:

sin² θ + cos² θ = 1

Rearrange:

1 − cos² θ = sin² θ

Worked Example

Simplify:

sec² θ − tan² θ

Use:

1 + tan² θ = sec² θ

Therefore:

sec² θ − tan² θ = 1

Worked Example

Simplify:

csc² θ − 1

Use:

1 + cot² θ = csc² θ

Therefore:

csc² θ − 1 = cot² θ

Recognising these patterns makes complicated expressions easier to simplify.

Combining Multiple Identities

More advanced problems often require several identities.

Worked Example

Simplify:

(1 − cos² θ)csc² θ

Start with:

(1 − cos² θ)csc² θ

Use the Pythagorean identity:

= sin² θ csc² θ

Use the reciprocal identity:

= sin² θ(1/sin² θ)

Cancel:

= 1

Valid where sin θ ≠ 0.

Worked Example

Simplify:

(sec² θ − 1)cos² θ

Use:

sec² θ − 1 = tan² θ

Then:

= tan² θ cos² θ

Use:

tan² θ = sin² θ/cos² θ

Substitute:

= (sin² θ/cos² θ)cos² θ

Simplify:

= sin² θ

Valid where cos θ ≠ 0.

Simplifying Complex Trigonometric Fractions

Fractions involving trigonometric functions can often be simplified by converting all functions to sine and cosine.

Worked Example

Simplify:

(tan θ + cot θ)/(sec θ csc θ)

Start with the numerator:

tan θ + cot θ

Replace both functions:

= sin θ/cos θ + cos θ/sin θ

Find a common denominator:

= (sin² θ + cos² θ)/(sin θ cos θ)

Apply the Pythagorean identity:

= 1/(sin θ cos θ)

Now simplify the denominator of the original expression:

sec θ csc θ

= (1/cos θ)(1/sin θ)

= 1/(sin θ cos θ)

Therefore, the complete expression becomes:

[1/(sin θ cos θ)]/[1/(sin θ cos θ)]

Simplify:

= 1

This result is valid where sin θ ≠ 0 and cos θ ≠ 0.

What Makes This Example Important?

It combines:

• Quotient identities.

• Common denominators.

• The Pythagorean identity.

• Reciprocal identities.

• Division of fractions.

A complicated expression can simplify to a single number when the correct identities are applied.

Factoring Trigonometric Expressions

Trigonometric expressions can often be simplified using ordinary algebra.

One particularly useful technique is the difference of squares:

a² − b² = (a − b)(a + b)

Worked Example

Simplify:

(1 − cos θ)(1 + cos θ)

Use the difference of squares:

= 1 − cos² θ

Apply the Pythagorean identity:

= sin² θ

Worked Example

Simplify:

(sec θ − tan θ)(sec θ + tan θ)

Use the difference of squares:

= sec² θ − tan² θ

Apply:

sec² θ − tan² θ = 1

Therefore:

= 1

Valid where cos θ ≠ 0.

Worked Example

Simplify:

sin⁴ θ − cos⁴ θ

Recognise a difference of squares:

= (sin² θ − cos² θ)(sin² θ + cos² θ)

Use:

sin² θ + cos² θ = 1

Therefore:

= (sin² θ − cos² θ)(1)

= sin² θ − cos² θ

Using the cosine double-angle identity, this can also be written:

= −cos 2θ

Using Conjugates to Simplify Expressions

A conjugate is formed by changing the sign between two terms.

For example:

1 + cos θ

and:

1 − cos θ

are conjugates.

Their product is:

(1 + cos θ)(1 − cos θ)

= 1 − cos² θ

= sin² θ

This makes conjugates useful in trigonometric simplification.

Worked Example

Simplify:

sin θ/(1 + cos θ)

Multiply numerator and denominator by:

1 − cos θ

Then:

= [sin θ(1 − cos θ)]/[(1 + cos θ)(1 − cos θ)]

Use the difference of squares:

= [sin θ(1 − cos θ)]/(1 − cos² θ)

Apply the Pythagorean identity:

= [sin θ(1 − cos θ)]/sin² θ

Cancel one factor of sine:

= (1 − cos θ)/sin θ

This is valid wherever the original expression is defined, requiring cos θ ≠ −1.

The transformed expression may be useful in further calculations.

Justifying Each Algebraic Step

When simplifying or proving identities, it is important to explain why each step is valid.

Consider:

(1 + tan² θ)cos² θ = 1

A complete justification can be written as follows.

Therefore:

(1 + tan² θ)cos² θ = 1

The identity is verified wherever cos θ ≠ 0.

What Makes a Step Valid?

Every transformation should follow from:

• A previously established identity.

• An algebraic rule.

• A definition.

• A valid operation on equivalent expressions.

For example:

sin² θ + cos² θ = 1

can be rearranged to:

sin² θ = 1 − cos² θ

because subtracting cos² θ from both sides preserves equality.

However, dividing by sin θ requires sin θ ≠ 0.

Constructing a Complete Identity Proof

A complete proof should begin with one side of the proposed identity and transform it into the other side.

Worked Example

Verify:

(1 − sin² θ)/cos θ = cos θ

Start with the left-hand side:

(1 − sin² θ)/cos θ

Use the Pythagorean identity:

= cos² θ/cos θ

Simplify:

= cos θ

Therefore:

(1 − sin² θ)/cos θ = cos θ

The identity is verified wherever cos θ ≠ 0.

Worked Example

Verify:

tan θ + cot θ = sec θ csc θ

Start with the left-hand side:

tan θ + cot θ

Use quotient identities:

= sin θ/cos θ + cos θ/sin θ

Find a common denominator:

= (sin² θ + cos² θ)/(sin θ cos θ)

Use the Pythagorean identity:

= 1/(sin θ cos θ)

Separate the factors:

= (1/cos θ)(1/sin θ)

Use reciprocal identities:

= sec θ csc θ

Therefore:

tan θ + cot θ = sec θ csc θ

The identity is verified wherever sin θ ≠ 0 and cos θ ≠ 0.

Advanced Proof: Combining Several Techniques

Verify:

(1 + sin θ)/cos θ = cos θ/(1 − sin θ)

Start with the left-hand side:

(1 + sin θ)/cos θ

Multiply numerator and denominator by:

1 − sin θ

Then:

= [(1 + sin θ)(1 − sin θ)]/[cos θ(1 − sin θ)]

Use the difference of squares:

= (1 − sin² θ)/[cos θ(1 − sin θ)]

Apply the Pythagorean identity:

= cos² θ/[cos θ(1 − sin θ)]

Cancel one factor of cosine:

= cos θ/(1 − sin θ)

Therefore:

(1 + sin θ)/cos θ = cos θ/(1 − sin θ)

The identity is verified wherever cos θ ≠ 0.

Mathematical Reasoning

This proof requires recognising that:

1. The numerator contains 1 + sin θ.

2. Multiplying by its conjugate creates 1 − sin² θ.

3. The Pythagorean identity converts this expression into cos² θ.

4. A common factor can then be cancelled.

The order of operations is important because each step prepares the expression for the next.

Recognising Appropriate Identity Substitutions

Choosing the correct identity is often the most challenging part of simplification.

The following patterns are especially useful.

A good problem-solver learns to recognise these structures rather than attempting random substitutions.

Domain Restrictions and Undefined Expressions

Trigonometric identities are only valid where their expressions are defined.

For example:

tan θ = sin θ/cos θ

requires:

cos θ ≠ 0

Similarly:

csc θ = 1/sin θ

requires:

sin θ ≠ 0

Common Restrictions

Why Restrictions Matter

Consider:

sin θ/sin θ

It may appear that this always equals 1.

However, if sin θ = 0, the expression becomes:

0/0

which is undefined.

Therefore:

sin θ/sin θ = 1 only when sin θ ≠ 0

Whenever a factor is cancelled, that factor must be nonzero.

Using Identities to Check Calculations

Trigonometric identities provide a way to check whether calculated values are consistent.

Suppose:

sin θ = 5/13

cos θ = 12/13

Check the Pythagorean identity:

sin² θ + cos² θ

= (5/13)² + (12/13)²

= 25/169 + 144/169

= 169/169

= 1

The values satisfy the identity.

Checking Tangent

tan θ = sin θ/cos θ

= (5/13)/(12/13)

= 5/12

We can also use the second Pythagorean identity.

Since:

sec θ = 13/12

then:

1 + tan² θ

= 1 + (5/12)²

= 1 + 25/144

= 169/144

And:

sec² θ = (13/12)² = 169/144

Both sides agree.

This provides an additional check on the calculations.

Applications in Physics: Resolving Forces

Trigonometric identities are useful when analysing forces acting at angles.

Suppose a force F acts at an angle θ above the horizontal.

Its components are:

Fₓ = F cos θ

Fᵧ = F sin θ

The magnitude of the force is related to its components by:

F² = Fₓ² + Fᵧ²

Substitute:

Fₓ² + Fᵧ²

= F²cos² θ + F²sin² θ

Factor:

= F²(cos² θ + sin² θ)

Apply the Pythagorean identity:

= F²(1)

Therefore:

Fₓ² + Fᵧ² = F²

This demonstrates that the trigonometric component formulas agree with the Pythagorean theorem.

Applications in Circular Motion

The coordinates of an object moving around a circle of radius r can be written:

x = r cos θ

y = r sin θ

To show that the object remains on the circle, calculate:

x² + y²

Substitute:

= r²cos² θ + r²sin² θ

Factor:

= r²(cos² θ + sin² θ)

Use the Pythagorean identity:

= r²

Therefore:

x² + y² = r²

This proves that the object remains at a constant distance r from the centre.

Applications in Waves and Oscillations

Sine and cosine functions are used to describe waves and oscillations.

For example:

y(t) = A sin(ωt)

where:

• A is the amplitude.

• ω is the angular frequency.

• t is time.

Trigonometric identities can simplify expressions involving wave displacement, velocity, and energy.

For example:

sin² θ + cos² θ = 1

is useful when combining squared sine and cosine terms.

Example: Oscillatory Energy

Suppose two energy quantities are represented by:

E₁ = E₀sin²(ωt)

E₂ = E₀cos²(ωt)

The total energy is:

E = E₁ + E₂

Substitute:

E = E₀sin²(ωt) + E₀cos²(ωt)

Factor:

E = E₀[sin²(ωt) + cos²(ωt)]

Apply the Pythagorean identity:

E = E₀

The total energy remains constant even though the two contributions vary with time.

A Strategy for Simplifying Complex Trigonometric Expressions

When simplifying a complicated expression:

1. Identify the trigonometric functions involved.

2. Look for reciprocal relationships.

3. Look for quotient identities.

4. Identify squared expressions that suggest Pythagorean identities.

5. Rewrite tangent, cotangent, secant, and cosecant in terms of sine and cosine when useful.

6. Combine fractions using common denominators.

7. Factor expressions when possible.

8. Consider multiplying by a conjugate.

9. Cancel common nonzero factors.

10. Check domain restrictions.

11. Justify important algebraic transformations.

12. Continue until the expression reaches a simpler or required form.

Choosing the Best Approach

If an expression contains many different trigonometric functions, converting everything to sine and cosine is often effective.

If it contains:

1 − cos² θ

a Pythagorean substitution is usually faster.

If it contains:

sec² θ − tan² θ

the expression simplifies directly to 1.

If it contains:

(1 + sin θ)(1 − sin θ)

the difference of squares is a useful first step.

Common Mistakes

Confusing reciprocal identities

Cosecant is the reciprocal of sine, not cosine.

Reversing quotient identities

Tangent is sine divided by cosine. Cotangent is cosine divided by sine.

Using the Pythagorean identity incorrectly

The correct identity is:

sin² θ + cos² θ = 1

not:

sin θ + cos θ = 1

Confusing sin² θ and sin 2θ

sin² θ means (sin θ)².

sin 2θ means sin(2θ).

Cancelling terms instead of factors

Cancellation is only valid for common nonzero factors.

Ignoring domain restrictions

Division by zero is undefined.

Taking only the positive square root

Quadrant information determines the sign of a trigonometric value.

Expanding expressions incorrectly

For example:

(1 + sin θ)(1 − sin θ) = 1 − sin² θ

not:

1 + sin² θ

Using too many unnecessary substitutions

Choose identities that move the expression toward a simpler form.

Failing to justify algebraic steps

Each transformation should follow from an identity or valid algebraic rule.

Assuming numerical examples prove an identity

Checking a few angles can detect errors but does not establish a general proof.

Forgetting to state the domain

A simplified expression may be defined at angles where the original expression was undefined.

Key Terms

Trigonometric Identity: An equation involving trigonometric functions that is true wherever both sides are defined.

Reciprocal Identity: An identity connecting a trigonometric function to its reciprocal.

Quotient Identity: An identity expressing tangent or cotangent using sine and cosine.

Pythagorean Identity: A trigonometric relationship derived from the Pythagorean theorem or unit circle.

Reciprocal: A number or expression obtained by exchanging the numerator and denominator of a nonzero fraction.

Quotient: The result of dividing one quantity by another.

Sine: The ratio of opposite side to hypotenuse in a right triangle.

Cosine: The ratio of adjacent side to hypotenuse in a right triangle.

Tangent: The ratio of opposite side to adjacent side, or sine divided by cosine.

Cosecant: The reciprocal of sine.

Secant: The reciprocal of cosine.

Cotangent: The reciprocal of tangent.

Pythagorean Theorem: The relationship a² + b² = c² in a right triangle.

Unit Circle: A circle of radius 1 centred at the origin.

Equivalent Expressions: Expressions that have the same value wherever both are defined.

Simplification: Rewriting an expression in a more useful or less complicated form.

Identity Proof: A logical demonstration that two expressions are equivalent.

Algebraic Justification: An explanation of the mathematical rule used in a transformation.

Substitution: Replacing an expression with an equivalent expression.

Factorisation: Rewriting an expression as a product of factors.

Difference of Squares: The algebraic relationship a² − b² = (a − b)(a + b).

Common Denominator: A shared denominator used when combining fractions.

Conjugate: An expression formed by changing the sign between two terms of a binomial.

Rationalisation: Rewriting a fraction by multiplying by a suitable expression, often a conjugate.

Cancellation: Removing common nonzero factors from a fraction.

Domain Restriction: A condition excluding values for which an expression is undefined.

Quadrant: One of the four regions of the Cartesian coordinate plane.

Reference Angle: The acute angle between an angle's terminal side and the x-axis.

Double-Angle Identity: An identity expressing a trigonometric function of twice an angle.

Vector Component: The part of a vector acting along a specified direction.

Periodic Function: A function whose values repeat at regular intervals.

Simple Harmonic Motion: Oscillatory motion that can be described using sine or cosine functions.

Check Your Understanding

1. Starting from the right-triangle definitions, derive the reciprocal identity csc θ = 1/sin θ.

2. Derive the reciprocal identity sec θ = 1/cos θ.

3. Use the definitions of sine and cosine to derive tan θ = sin θ/cos θ.

4. Derive cot θ = cos θ/sin θ.

5. Starting from the Pythagorean theorem, derive sin² θ + cos² θ = 1.

6. Derive 1 + tan² θ = sec² θ by dividing the fundamental Pythagorean identity by cos² θ.

7. Derive 1 + cot² θ = csc² θ by dividing the fundamental Pythagorean identity by sin² θ.

8. Given sin θ = 5/13 and θ lies in Quadrant II, determine cos θ, tan θ, and sec θ.

9. Simplify:

sin θ csc θ + cos θ sec θ

10. Simplify:

tan θ cos θ + cot θ sin θ

11. Simplify:

(sec² θ − 1)cos² θ

12. Verify:

tan θ + cot θ = sec θ csc θ

13. Simplify:

(1 − cos θ)(1 + cos θ)

14. Verify:

(1 + tan² θ)cos² θ = 1

Explain each algebraic step.

15. Construct a complete proof of:

(1 + sin θ)/cos θ = cos θ/(1 − sin θ)

Your proof should include identity substitutions, algebraic justifications, domain restrictions, and a concluding statement.

Key Takeaways

• Fundamental trigonometric identities are relationships that can be derived from right-triangle definitions, the Pythagorean theorem, and the unit circle.

• Reciprocal identities connect sine with cosecant, cosine with secant, and tangent with cotangent.

• The reciprocal identities follow from taking the reciprocals of trigonometric ratios.

• The tangent quotient identity is tan θ = sin θ/cos θ.

• The cotangent quotient identity is cot θ = cos θ/sin θ.

• Quotient identities can be derived by dividing the right-triangle definitions of sine and cosine.

• The fundamental Pythagorean identity is sin² θ + cos² θ = 1.

• This identity follows directly from the Pythagorean theorem and the unit circle.

• Dividing the fundamental Pythagorean identity by cos² θ produces 1 + tan² θ = sec² θ.

• Dividing by sin² θ produces 1 + cot² θ = csc² θ.

• Pythagorean identities can be rearranged to produce several useful equivalent relationships.

• Trigonometric identities allow unknown function values to be calculated without finding the angle itself.

• Quadrant information is important when determining the signs of trigonometric values.

• Reciprocal identities are useful for simplifying products.

• Quotient identities allow complicated expressions to be rewritten using sine and cosine.

• Pythagorean identities simplify expressions involving squared trigonometric functions.

• Complex expressions often require several identities and algebraic techniques.

• Common denominators, factoring, and conjugates are valuable tools in trigonometric simplification.

• Every algebraic transformation should be justified using an established identity or valid mathematical operation.

• Domain restrictions must be respected whenever division or cancellation occurs.

• Trigonometric identities are useful in force calculations, circular motion, waves, and oscillations.

• Understanding how identities are derived, rather than simply memorising them, develops stronger mathematical reasoning and prepares students for advanced trigonometry and calculus.