Question

Verify as an identity: cos(90°+x) = -sinx I'm not sure how to go about this question

Answer 1

Cos( a + b ) = cosa×cosb - sina×sinb ; 
a = 90 and b = x ;
But, cos90 = 0 and sin90 = 1;
cos( 90 + x ) = 0 × cosx - 1 × sinx = - sinx ;