First slide

Compound angles

Question

If $\cos α + \cos β = 0 = s i n α + s i n β,$ then $\cos 2 α + \cos 2 β$ is equal to

Moderate

A

$- 2 \sin (α + β)$
B

$- 2 \cos (α + β)$
C

$2 \sin (α + β)$
D

$2 \cos (α + β)$

Solution

${(\cos α + \cos β)}^{2} - {(\sin α + \sin β)}^{2} = 0$

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