Question

prove that cos100 cos 40+sin 100 sin 40 =1/2​
WITH EXPLANATION PLEASE HELP ME​

Answer 1

LHS = cos 100° .cos40° + sin 100° .sin40 °

Using cos (a-b) = cos a cos b + sin a sin b

= cos(100° - 40°)

= cos(60°)

= 1/2 = RHS