Evaluate : `int_(0)^(pi//2)cos^(3)x\ dx`

Explanation

To evaluate the integral \( I = \int_0^{\frac{\pi}{2}} \cos^3 x \, dx \), we can use the trigonometric identity for \( \cos^3 x \).

### Step 1: Use the identity for \( \cos^3 x \)

We know that:
\[ \cos^3 x = \frac{1}{4} (3 \cos x + \cos 3x) \]
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