Question

Cos2(125-x)-cos2(55+x)

Answer 1

cos² (125 - x) - cos² (55 + x)

= cos² {90 + (35 - x)} - cos² {90 - (35 - x)}

= sin² (35 - x) - sin² (35 - x)

= 0

RULE :

sin (90 - θ) = cosθ

cos (90 + θ) = - sinθ

cos (90 - θ) = sinθ