Prove that: `sin(pi/10) sin((13pi)/10)=-1/4`

Explanation

To prove that \( \sin\left(\frac{\pi}{10}\right) \sin\left(\frac{13\pi}{10}\right) = -\frac{1}{4} \), we can follow these steps:

### Step 1: Rewrite the sine function
We start with the expression \( \sin\left(\frac{13\pi}{10}\right) \). We can express this angle in terms of \( \pi \):
\[ \sin\left(\frac{13\pi}{10}\right) = \sin\left(\pi + \frac{3\pi}{10}\right) \]
Using the sine addition formula, we know that:
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