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