Francis

Mathematics

12 June, 04:07

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