Proving identities using the addition formulae

You will be expected to be able to prove a trig. identity such as the examples below. In the videos I show you how to set out an identity and what to look for.

This is a tricky topic and one that I find students give in too quickly. Learn your formulae and have patience.

Addition Formulae (Sum and Difference Formulae):

  • sin (A + B) ≡ sin A cos B + cos A sin B
    sin (A – B) ≡ sin A cos B – cos A sin B
    cos (A + B) ≡ cos A cos B – sin A sin B
    cos (A – B) ≡ cos A cos B + sin A sin B
    tan (A + B) ≡ (tan A + tan B) / (1 – tan A tan B)
    tan (A – B) ≡ (tan A – tan B) / (1 + tan A tan B)

Example 1

Example 2

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