Prove that `4cos((2pi)/7).cos(pi/7)-1=2cos((2pi)/7)`.

Explanation

To prove the equation \( 4 \cos\left(\frac{2\pi}{7}\right) \cos\left(\frac{\pi}{7}\right) - 1 = 2 \cos\left(\frac{2\pi}{7}\right) \), we can follow these steps:

### Step 1: Rewrite the left-hand side
Start with the left-hand side of the equation:
\[ 4 \cos\left(\frac{2\pi}{7}\right) \cos\left(\frac{\pi}{7}\right) - 1 \]

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