#### Answer

Write an expression for $P$ that involves the cosine function.
$$P=16k\cos^2(2\pi t)$$

#### Work Step by Step

$$y=4\cos(2\pi t)$$
$$P=ky^2$$
Write an expression for $P$ that involves the cosine function.
We replace the formula of $y$ into the formula of $P$.
$$P=ky^2$$
$$P=k[4\cos(2\pi t)]^2$$
$$P=k[16\cos^2(2\pi t)]$$
$$P=16k\cos^2(2\pi t)$$