The easiest way to solve anything in this form is to use the identity cos(A+B)+cos(A-B).
sorry, could you just do a quick solution using that identity? was never actually taught this in p2 and kinda hoped i wouldnt need these identities for p3 :]
sorry, could you just do a quick solution using that identity? was never actually taught this in p2 and kinda hoped i wouldnt need these identities for p3 :]
cosA+cosB = 2cos(A+B/2)cos(A-B/2)
Equate to cos4xcos5x and you get
4x = A+B/2, 5x = A-B/2 and cos4xcos5x = 1/2(cosA+cosB)