First slide

Trigonometric Identities

Question

If $\cos (x - y) = - 1,$ then $\cos x + \cos y =$

Moderate

A

0
B

1
C

2
D

3

Solution

$x - y = π$
$∴ \cos x + \cos y = \cos (π + y) + \cos y = - \cos y + \cos y = 0$

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