Question

cos60°-cos20°=2sin40°sin20°

Answer 1

In this question use this formula :-

cosA - cosB = -2sin(A+B)/2 sin(A-B)/2

OR

= 2sin(A+B)/2 sin(B-A)/2

use anyone of them, the answer will be same

cos60° - cos20° = -2sin(60+20)/2 sin(60-20)/2

= -2 sin(80/2) sin(40/2)

= -2sin40°sin20°

check your question again

I think it is -2sin40°sin20