put x= -x,

f(sin(-x)) = f(cos(-x)).

so, f(-Sinx)) = f(cosx)

we get, f(-Sinx)) = f(Sinx)

It implies, it can be true only when f(x) is constant, beacuse it's giving f(-1/2) =f(1/2) and f(-1) =f(1)

and so on...

Possible for constant function only. => f(x) =c.