Evaluate using multiple angle formula: `cos15^@`

Explanation

To evaluate \( \cos 15^\circ \) using the multiple angle formula, we can follow these steps:

### Step 1: Use the Cosine Double Angle Formula
The cosine double angle formula states:
\[ \cos 2\theta = 2\cos^2 \theta - 1 \]
We want to express \( \cos 15^\circ \) in terms of \( \cos 30^\circ \) because \( 15^\circ \) is half of \( 30^\circ \). Thus, we set \( \theta = 15^\circ \), which gives us:
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