Well, start by writing out a table for 0-pi/2 for sine and cosine.

Sin(x)

sin(0) = 0

sin(pi/6) = 1/2

sin(pi/4) = 1/root(2)

sin(pi/3) = root(3)/2

sin(pi/2) = 1

Cos(x)

cos(0) = 1

cos(pi/6) = root(3)/2

cos(pi/4) = 1/root(2)

cos(pi/3) = 1/2

cos(pi/2) = 0

Notice anything in particular about them?