cos(4deg)cos(9deg)+cos(86deg)cos(81deg)

Question

Simply by applying odd/even identities, cofunction identities, and cosine of sum or difference identities.

Answer 1

if E=cos(4deg)cos(9deg)+cos(86deg)cos(81deg)

then E=(1/2)*[cos(13)+cos(5)+cos(167)+cos(5)]

cos(167)= -cos(13)

E=cos(5)

Answer 2

cos4°cos9° + cos86°cos81° = cos4°cos9° + sin4°sin9° = cos(9°- 4°) = cos5°