Prove that: `(cosx-cosy)^2+(sinx-siny)^2=4sin^2``(x-y)/2`

Explanation

To prove the identity \((\cos x - \cos y)^2 + (\sin x - \sin y)^2 = 4 \sin^2\left(\frac{x - y}{2}\right)\), we will follow these steps:

### Step 1: Expand the left-hand side
We start with the left-hand side of the equation:

\[ (\cos x - \cos y)^2 + (\sin x - \sin y)^2 \]
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