`costheta-cos2theta=sin3theta`.Then find its general solution

Explanation

To solve the equation \( \cos \theta - \cos 2\theta = \sin 3\theta \), we will follow these steps:

### Step 1: Use the Cosine Difference Identity
We can use the identity for the difference of cosines:
\[ \cos A - \cos B = -2 \sin\left(\frac{A + B}{2}\right) \sin\left(\frac{A - B}{2}\right) \]
In our case, let \( A = \theta \) and \( B = 2\theta \). Thus, we rewrite the left-hand side:
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