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
