Advanced Trig Identities — AHL 3.7 Further trigonometric identities (compound angles; product-to-sum) (HL) - IB Mathematics Analysis and Approaches

SECTION 1

Historical Context & Motivation

For thousands of years, astronomers and mathematicians wrestled with a fundamental question: how do you compute the sine or cosine of an angle that is built from two smaller angles? Ancient Greek astronomers needed to calculate chord lengths in circles to predict planetary positions, but they lacked a clean formula to combine angles. The development of compound-angle identities solved this problem elegantly and opened the door to centuries of mathematical progress.

~150 CE • Ptolemy's Chord Tables

Claudius Ptolemy develops a formula equivalent to the modern compound-angle identity for cosine in his Almagest, using chord lengths instead of sine and cosine.

~1400 • Islamic Golden Age Refinements

Mathematicians such as Jamshīd al-Kāshī refine trigonometric tables to remarkable precision, using sum-and-difference formulas to compute sines of angles down to fractions of a degree.

1748 • Euler Connects Trig to Complex Numbers

Leonhard Euler publishes Introductio in analysin infinitorum, showing that compound-angle formulas follow naturally from e^iθ = cos θ + i sin θ, unifying trigonometry with exponential functions.

1822 • Fourier's Heat Equation

Joseph Fourier uses product-to-sum identities to decompose complex waveforms into sums of sine and cosine terms, founding the field of Fourier analysis used in modern signal processing.

Today, these identities are essential tools in the IB HL curriculum. They let you expand expressions like sin(A + B), simplify products of trig functions into sums, and solve equations that would otherwise be intractable. The central question this topic addresses is: how can we break apart or recombine trigonometric expressions involving multiple angles?

SECTION 2

Core Principles & Definitions

Before diving into formulas, it helps to understand the key ideas that underpin all of the identities in this topic. Each identity is not an isolated formula to memorize — they all connect to a few fundamental principles about how angles combine and how trig functions relate to each other.

Compound Angles

A compound angle is formed by adding or subtracting two angles, such as (A + B) or (A − B). The sine or cosine of a compound angle is NOT simply the sum of the individual sines or cosines.

Double-Angle Identities

Setting B = A in the compound-angle formulas gives the double-angle identities for sin 2A, cos 2A, and tan 2A. These are special cases, not separate formulas.

Product-to-Sum Conversion

Product-to-sum formulas convert a product like sin A · cos B into a sum (or difference) of sine/cosine terms. This transformation simplifies integration and equation-solving.

Sum-to-Product Conversion

The reverse process, sum-to-product formulas, converts sums like sin A + sin B into products. These are useful for solving equations and proving other identities.

Identity vs. Equation

An identity is true for all values of the variable (where defined). An equation is true only for specific values. Every formula in this section is an identity.

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KEY TAKEAWAY

Think of compound-angle formulas like a recipe for a smoothie. You can't figure out the taste of a strawberry-banana smoothie by just adding 'strawberry flavor' and 'banana flavor' separately — the ingredients interact. Similarly, sin(A + B) ≠ sin A + sin B. The compound-angle formula tells you exactly how the two angles 'mix' together to produce the final result.

SECTION 3

Visual Explanation — The Geometry of Compound Angles

The compound-angle formula for sine can be understood geometrically using the unit circle. The following diagram shows how sin(A + B) arises from projecting lengths in two rotated right triangles. Study the colored segments carefully — each one corresponds to a term in the formula.

The pink vertical segment represents sin(A + B), while the green horizontal segment represents cos(A + B). The total angle (A + B) is formed by first rotating by angle A (cyan) and then by angle B (violet).

Notice that the point on the unit circle at angle (A + B) has coordinates (cos(A + B), sin(A + B)). The compound-angle formulas tell us how to express these coordinates in terms of sin A, cos A, sin B, and cos B individually. By constructing perpendicular projections inside the diagram, each term in the formula — like sin A cos B and cos A sin B — corresponds to a specific geometric length.

SECTION 4

Mathematical Framework — The Key Identities

This section presents every identity you need for AHL 3.7. Each formula is given in the IB formula booklet, but understanding what each part means will help you use them fluently. We begin with the compound-angle (addition) formulas, then derive double-angle formulas from them, and finally present the product-to-sum and sum-to-product identities.

Compound-Angle (Addition) Formulas

SINE ADDITION

sin(A + B) = sin A cos B + cos A sin B

A and B are any real angles. This expands the sine of a sum into four trig values of the individual angles.

SINE SUBTRACTION

sin(A − B) = sin A cos B − cos A sin B

Replace B with −B in the addition formula. Since cos(−B) = cos B and sin(−B) = −sin B, the second term flips sign.

COSINE ADDITION

cos(A + B) = cos A cos B − sin A sin B

Note the minus sign between the two products — this is easy to confuse with the sine formula.

COSINE SUBTRACTION

cos(A − B) = cos A cos B + sin A sin B

The subtraction version flips the middle sign to a plus.

TANGENT ADDITION

tan(A + B) = (tan A + tan B) / (1 − tan A tan B)

Derived by dividing sin(A + B) by cos(A + B). Undefined when tan A tan B = 1.

Double-Angle Identities

DOUBLE-ANGLE SINE

sin 2A = 2 sin A cos A

Set B = A in sin(A + B). This identity appears frequently in calculus and optimization problems.

DOUBLE-ANGLE COSINE (three forms)

cos 2A = cos²A − sin²A = 2cos²A − 1 = 1 − 2sin²A

The first form comes directly from cos(A + A). The other two follow by substituting sin²A = 1 − cos²A or cos²A = 1 − sin²A.

Product-to-Sum Formulas

These four formulas are derived by adding or subtracting the compound-angle identities. They convert a product of two trig functions into a sum or difference, which is often easier to integrate or simplify.

PRODUCT TO SUM — sin × cos

sin A cos B = ½[sin(A + B) + sin(A − B)]

Add the sin(A + B) and sin(A − B) formulas, then divide by 2.

PRODUCT TO SUM — cos × cos

cos A cos B = ½[cos(A − B) + cos(A + B)]

Add the cos(A + B) and cos(A − B) formulas, then divide by 2.

PRODUCT TO SUM — sin × sin

sin A sin B = ½[cos(A − B) − cos(A + B)]

Subtract cos(A + B) from cos(A − B), then divide by 2. Note the result involves cosines, not sines.

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IB Exam Tip

The compound-angle and double-angle formulas are in your formula booklet, but the product-to-sum formulas may not be listed explicitly. However, you can always derive them quickly by adding or subtracting the compound-angle formulas. Practice this derivation until you can do it in under 60 seconds.

SECTION 5

Sum-to-Product Identities & Classification

The sum-to-product identities are the reverse of the product-to-sum formulas. They convert a sum (or difference) of two trig functions into a product. These are especially powerful when solving equations of the form sin X + sin Y = 0 or when factoring trigonometric expressions.

SUM TO PRODUCT — sin + sin

sin P + sin Q = 2 sin((P + Q)/2) cos((P − Q)/2)

Let A + B = P and A − B = Q, so A = (P + Q)/2 and B = (P − Q)/2. Substitute into the product-to-sum formula for sin A cos B.

SUM TO PRODUCT — sin − sin

sin P − sin Q = 2 cos((P + Q)/2) sin((P − Q)/2)

Similar derivation but using the subtraction of the compound-angle sine formulas.

SUM TO PRODUCT — cos + cos

cos P + cos Q = 2 cos((P + Q)/2) cos((P − Q)/2)

Derived from adding cos(A + B) and cos(A − B) with the same substitution.

SUM TO PRODUCT — cos − cos

cos P − cos Q = −2 sin((P + Q)/2) sin((P − Q)/2)

Note the negative sign in front. This often catches students off guard on exams.

This map shows how every identity family in AHL 3.7 derives from the compound-angle formulas at the top. The double-angle formulas are a special case (B = A). The product-to-sum and sum-to-product families are inverses of each other.

Direction	Input Form	Output Form	When to Use
Product → Sum	sin A cos B	½[sin(A+B) + sin(A−B)]	Integrating products, simplifying expressions
Sum → Product	sin P + sin Q	2 sin((P+Q)/2) cos((P−Q)/2)	Solving trig equations, factoring
Expand	sin(A + B)	sin A cos B + cos A sin B	Finding exact values, proving identities
Condense	2 sin A cos A	sin 2A	Simplifying, solving double-angle equations

Quick reference: which direction to transform your expression

SECTION 6

Worked Example — Finding an Exact Value

Let's put the compound-angle formulas to work. We'll find the exact value of sin 75° without a calculator, and then use a product-to-sum identity to simplify a product of trig functions.

Example 1: Find the exact value of sin 75°

1
Step 1 — Decompose the angle

Recognize that 75° = 45° + 30°. Both 45° and 30° are standard angles whose sine and cosine values we know exactly.

75° = 45° + 30°
2
Step 2 — Apply sin(A + B)

Using sin(A + B) = sin A cos B + cos A sin B with A = 45° and B = 30°: sin 75° = sin 45° cos 30° + cos 45° sin 30°

sin 75° = sin 45° cos 30° + cos 45° sin 30°
3
Step 3 — Substitute known values

Recall: sin 45° = √2/2, cos 30° = √3/2, cos 45° = √2/2, sin 30° = 1/2. Substituting: sin 75° = (√2/2)(√3/2) + (√2/2)(1/2)

sin 75° = (√6/4) + (√2/4)
4
Step 4 — Simplify

Combine over a common denominator:

sin 75° = (√6 + √2) / 4 ≈ 0.9659

Example 2: Express sin 5x cos 3x as a sum

1
Step 1 — Identify the formula

This is a product of sin and cos, so we use: sin A cos B = ½[sin(A + B) + sin(A − B)]. Here A = 5x and B = 3x.
2
Step 2 — Substitute

sin 5x cos 3x = ½[sin(5x + 3x) + sin(5x − 3x)]

= ½[sin 8x + sin 2x]
3
Step 3 — Interpret

The product of sin 5x and cos 3x has been rewritten as the average of two sine functions. This form is much easier to integrate or to analyze for frequency content.

sin 5x cos 3x = ½ sin 8x + ½ sin 2x

SECTION 7

Strengths, Limitations & Common Mistakes

These identities are powerful tools, but students often stumble on a few predictable mistakes. Understanding both the strengths and the pitfalls will save you marks on exams and build deeper fluency.

Strength	Limitation / Pitfall
Allows exact values for non-standard angles (15°, 75°, etc.)	Only works if you can decompose the angle into known standard angles
Converts products to sums, making integration straightforward	Students often confuse which formula applies (sin×sin vs. sin×cos)
Enables solving complex trig equations by factoring	The cos(A − B) vs. cos(A + B) sign confusion is very common
Three forms of cos 2A give flexibility in choosing the right one	Choosing the wrong form can make simplification harder, not easier
All identities derive from compound-angle formulas, so you can re-derive anything you forget	Re-derivation takes time under exam pressure — practice until recall is fast

Strengths and common pitfalls of trigonometric identities

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Common Mistake Alert

The most frequent error on IB exams is writing sin(A + B) = sin A + sin B. This is WRONG. Quick check: sin(30° + 60°) = sin 90° = 1, but sin 30° + sin 60° = 0.5 + 0.866 = 1.366. They are clearly not equal. Always use the full compound-angle formula.

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KEY TAKEAWAY

Think of the sign patterns as a mnemonic: for sine formulas, the sign in the middle matches the sign of the compound angle (sin(A + B) has a +, sin(A − B) has a −). For cosine formulas, the sign in the middle is the opposite of the compound angle sign (cos(A + B) has a −, cos(A − B) has a +). This 'same sign for sine, opposite sign for cosine' rule prevents the most common errors.

SECTION 8

Connection to Advanced Theory

The compound-angle and product-to-sum identities are not the end of the road — they serve as the foundation for several powerful ideas you'll encounter if you continue in mathematics, physics, or engineering. Understanding these connections now gives you a preview of why these identities are so important beyond the IB exam.

AHL 3.7 Identity	Advanced Extension	Where You'll Meet It
sin(A + B), cos(A + B)	Euler's formula: $e^{iθ}$ = cos θ + i sin θ	University complex analysis, electrical engineering
Product-to-sum identities	Fourier transforms: decomposing signals into frequencies	Signal processing, acoustics, image compression (JPEG)
Double-angle formulas	Power-reduction formulas for integration	IB Calculus (AHL 5.16), university-level integration
Sum-to-product identities	Beat frequencies in wave interference	IB Physics (wave phenomena), music theory

How AHL 3.7 identities connect to advanced topics

One beautiful connection worth noting: when you add two sound waves with slightly different frequencies, say sin(2π × 440t) and sin(2π × 442t), the sum-to-product formula converts this into 2 cos(2π × 1 × t) sin(2π × 441t). The cosine factor creates a slow 'beating' effect at 2 Hz while the sine factor plays at 441 Hz. This is exactly what musicians hear as beat frequencies when tuning instruments — a direct, audible consequence of sum-to-product identities.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A student claims that cos(A + B) = cos A + cos B. Without performing a calculation, explain why this claim must be false, and give a specific pair of angle values that disproves it.

Show answer

The claim is false because trigonometric functions are nonlinear — they do not distribute over addition. A simple counterexample: let A = 60° and B = 60°. Then cos(60° + 60°) = cos 120° = −0.5, but cos 60° + cos 60° = 0.5 + 0.5 = 1. Since −0.5 ≠ 1, the claim is disproved. The correct identity is cos(A + B) = cos A cos B − sin A sin B, which accounts for how the angle components interact multiplicatively.

Question 2: BASIC CALCULATION

Find the exact value of cos 15° using the compound-angle formula cos(A − B) = cos A cos B + sin A sin B.

Show answer

Write 15° = 45° − 30°. Then cos 15° = cos 45° cos 30° + sin 45° sin 30° = (√2/2)(√3/2) + (√2/2)(1/2) = √6/4 + √2/4 = (√6 + √2)/4. This equals approximately 0.9659. Notice this is the same value as sin 75°, which makes sense because cos θ = sin(90° − θ), and 90° − 15° = 75°.

Question 3: INTERMEDIATE

Prove the identity: (sin 3x − sin x) / (cos 3x + cos x) = tan x.

Show answer

Apply sum-to-product formulas to numerator and denominator separately. Numerator: sin 3x − sin x = 2 cos((3x + x)/2) sin((3x − x)/2) = 2 cos 2x sin x. Denominator: cos 3x + cos x = 2 cos((3x + x)/2) cos((3x − x)/2) = 2 cos 2x cos x. Dividing: (2 cos 2x sin x)/(2 cos 2x cos x) = sin x / cos x = tan x, which completes the proof. The key insight was recognizing that both the numerator and denominator share the common factor 2 cos 2x, which cancels.

Question 4: APPLIED

Two speakers emit sound waves modeled by y₁ = sin(2π × 500t) and y₂ = sin(2π × 504t), where t is in seconds. Use a sum-to-product formula to express y₁ + y₂ as a product, and determine the beat frequency.

Show answer

Using sin P + sin Q = 2 sin((P + Q)/2) cos((P − Q)/2), let P = 2π × 500t and Q = 2π × 504t. Then (P + Q)/2 = 2π × 502t and (P − Q)/2 = 2π × (−2)t. So y₁ + y₂ = 2 sin(2π × 502t) cos(2π × 2t). The sin term oscillates at the average frequency 502 Hz (the perceived pitch), while the cos term oscillates at 2 Hz, creating an amplitude envelope. Since the listener hears the amplitude peak twice per cycle of the cosine, the beat frequency is |500 − 504| = 4 Hz. This is how musicians detect that two notes are slightly out of tune.

Question 5: CRITICAL THINKING

Given that sin(A + B) = sin A cos B + cos A sin B and cos(A + B) = cos A cos B − sin A sin B, derive the product-to-sum formula for sin A sin B entirely from scratch. Show every step and explain why you add or subtract the two compound-angle identities.

Show answer

Start with the cosine subtraction and addition formulas: (i) cos(A − B) = cos A cos B + sin A sin B, and (ii) cos(A + B) = cos A cos B − sin A sin B. We want to isolate sin A sin B, so we need to eliminate the cos A cos B terms. Subtract equation (ii) from equation (i): cos(A − B) − cos(A + B) = (cos A cos B + sin A sin B) − (cos A cos B − sin A sin B) = 2 sin A sin B. Therefore sin A sin B = ½[cos(A − B) − cos(A + B)]. The strategy was to subtract rather than add because the cos A cos B terms have the same sign in both equations, so subtraction cancels them. Had we added instead, we would have eliminated sin A sin B and derived the cos A cos B product-to-sum formula. This shows that each product-to-sum formula comes from a deliberate choice of adding or subtracting the right pair of compound-angle identities.

SUMMARY

Lesson Summary

Summary

The compound-angle formulas — sin(A ± B), cos(A ± B), and tan(A ± B) — are the foundation of AHL 3.7. They express the sine, cosine, or tangent of a sum or difference of two angles in terms of the trig functions of each angle individually. From these, the double-angle identities (sin 2A = 2 sin A cos A; cos 2A in three forms) emerge as special cases when B = A. Remember the sign rule: for sine formulas, the middle sign matches the compound angle sign; for cosine formulas, it is the opposite.

The product-to-sum identities convert products like sin A cos B into sums of sine or cosine terms, and the sum-to-product identities reverse this process, turning sums into factorable products. Both families are derived by adding or subtracting pairs of compound-angle formulas. These tools allow you to find exact values of non-standard angles, prove identities, simplify expressions for integration, solve complex trig equations, and even model physical phenomena like beat frequencies in sound waves.