Help me with my trig homework please?

cos(a-b) = cos(a)cos(b) + sin(a)sin(b)
In this case a is pi/2 and b is x so,

cos(pi/2 - x) = cos(pi/2)cos(x) + sin(pi/2)sin(x)
= 0*cos(x) + 1*sin(x)
=sin(x)

Use the angle addition formula for cosines.

cos(a - b) = (cos a)(cos b) + (sin a)(sin b)

Do you mean cos((pi/2)-x) or cos(pi/(2-x))?

pay attention in class or get a tutor. We won't give you the answer for your homework.