Evaluate the following definite integral: `int_0^(pi//6)cosxcos2x\ dx`

Explanation

Let `I=\int_{0}^{\pi / 6} \cos x \cos 2 x d x`


`\cos A \cos B=\frac{1}{2}[\cos (A+B)+\cos (A-B)]`



`I=\int_{0}^{\pi / 6} \frac{1}{2}[\cos 3 x+\cos x] d x `


`I=\frac{1}{2}[\int_{0}^{\pi / 6} \cos 3 x d x+\int_{0}^{\pi / 6} \cos x d x] `
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