Using addition formulas for sine and cosine to simplify the expression:

cos (2pi/9) cos (pi/18) sin (2pi/9) sin(pi/18)

i want to learn it the normal way first before i add my instructors instructions to place s=45 & t = 30.

here are the formulas given:

sin(s t) = sin s cos t cos s sine t
sin(s-t) = sin s cos t - cos s sine t
cos(s t) = cos s cos t - sin s sin t
cos(s-t) = cos s cos t sin s sin t

the they way it looks, it seems to use formula 4:
cos(s-t) = cos s cos t sin s sin t