Question

cos(pi/5)cos(pi/2)-sin(pi/5)sin(pi/2)

Answer 1

Look at angle sum formula.

Answer 2

\[
\cos(x+y) = \cos(x)\cos(y) - \sin(x)\sin(y)
\]

Answer 3

cos(x) * sin(y) = 1/2 * (sin(x+y) - sin(x - y))

sin(x) * sin(y) = 1/2 * (cos(x-y) - cos(x+y))

Answer 4

Or wait, you can also use the fact that \(\cos(\pi/2) - 0\) and \(\sin(\pi/2) = 1\).

Answer 5

those product identities I posted should help you with this.