Trigonometryhard · Past Paper
If cos A + cos B = 0 and sin A + sin B = 0, then cos(A-B) is:
A-1
B1
C0
D1/2
✓ Correct Answer: A — -1
Squaring and adding: (cos A + cos B)^2 + (sin A + sin B)^2 = 0 => 2 + 2 cos(A-B) = 0 => cos(A-B) = -1.
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