Find the value of
`cos^2. pi/16+cos^2. (3pi)/16+cos^2. (7pi)/16`.

Explanation

To find the value of

\[ \cos^2\left(\frac{\pi}{16}\right) + \cos^2\left(\frac{3\pi}{16}\right) + \cos^2\left(\frac{7\pi}{16}\right), \]

we can use some trigonometric identities and properties.

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