Question

Rewrite cos2 67.5° – sin2 67.5° using a double-angle identity.
cos 135°

sin 135°

cos 33.75°

sin 33.75°

Answer 1

The Double Angle identity for cos is as follows:
$cos(2a)=cos^2(a)-sin^2(a)$
Because this is the same form as the equation you asked for, just double 67.5 to get cos(135).