Paper 2 · Trigonometry Lesson 8 of 13

Trigonometry 4.7 – Trig Proof 3

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You’ll learn

Prove the cosine rule by placing vertex A at the origin of a circle of radius c
Show a point at angle A on a radius-c circle has coordinates (c cos A, c sin A)
Apply the distance formula from B(b, 0), expand, and close with cos²A + sin²A = 1

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