Question

If A = 60° and B = 30°, verify that cos (A + B) = cos A cos B – sin A sin B.

Answer 1

cos (A + B) = cos 90⁰ = 0  
cos A cos B - sin A sin B = cos 60⁰ cos 30⁰ - sin 60⁰ sin 30⁰   

=`(1/2 xx sqrt(3)/2 - sqrt(3)/2 xx1/2) = ( sqrt(3)/4 - sqrt(3)/4) = 0`

∴ cos (A + B) = cos A cos B - sin A sin B