Use identities to find the value of the expression.

Question

If sin(x)=0.49, find cos(pi/2-x)

Answer 1

One of the trig identities you typically learn is that cos(π/2-x) = sin(x). So in your problem cos(π/2-x) = .49

But if you don't remember that one, you can use the cos(u-v) = cos u cos v + sin u sin v.

So cos(π/2-x) = cos π/2 cos x + sin π/2 sin x. Conveniently, cos π/2 = 0. So, cos(π/2-x) = 0 cos x + 1 sin x = sin x.