Paper 2 · Trigonometry
⏱ Tested in 2 exam questions since 2023
Lesson 9 of 13
Trigonometry 4.8 – Trig Proof 4
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- Prove cos(A − B) = cos A cos B + sin A sin B — the hardest of the eight proofs
- Place P(cos A, sin A) and Q(cos B, sin B) on the unit circle with angle A − B at O
- Compute |PQ|² two ways — cosine rule and distance formula — and equate them
- Collapse the algebra by applying the Pythagorean identity twice
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