Cos10°- 2cos50°- cos70°

Question

Cos10°- 2cos50°- cos70°

Answer 1

=(cos10-cos70)-2cos50=-2sin(10+70)/2*sin(10-70)/2-2cos50=-2sin40*sin(-30)-2sin40=
=-2sin40(1+1/2)=-3/2*2sin40=-3sin40