Solve: `cos4x=cos2x`

Explanation

To solve the equation \( \cos(4x) = \cos(2x) \), we can use the property of cosine that states if \( \cos A = \cos B \), then:

1. \( A = B + 2n\pi \) (for some integer \( n \))
2. \( A = -B + 2n\pi \)

In this case, we can set \( A = 4x \) and \( B = 2x \).

### Step 1: Set up the equations
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