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