∫ cos mx cos nx dx, m ≠ n
We know 2cosAcosB = cos(A – B) + cos(A + B)
∴ cosmxcosnx = 
∴ The above equation becomes
⇒ 
We know 
⇒ 
⇒ 
3
∫ cos mx cos nx dx, m ≠ n
We know 2cosAcosB = cos(A – B) + cos(A + B)
∴ cosmxcosnx =
∴ The above equation becomes
⇒
We know
⇒
⇒