Calculate cos120 and sin15 using the sum andor difference

. Calculate cos(120 ) and sin(15 ) using the sum and/or difference formulas.

cos (120)=cos(90+30)

This is of the form cos(A+B)= cos A cos B - sinA sinB

cos (90+30)=cos 90 cos 30 - sin 90 sin 30= 0* sqrt3/2 - 1*1/2=-1/2

sin 15=sin(45-30)

sin(A-B)= sinA cos B - cos A sin B

sin(45-30)=sin 45 cos 30-cos 45 sin 30=(sqrt2/2)(sqrt3/2)-(sqrt2/2)(1/2)= (sqrt6 - sqrt2)/4