Answer
Write $I$ in terms of the sine function:
$$I=k-k\sin^2\theta$$
Work Step by Step
$$I=k\cos^2\theta$$
The exercise asks to write $I$ in terms of the sine function. This can be done by changing $\cos\theta$ into $\sin\theta$.
Using Pythagorean Identity: $$\cos^2\theta=1-\sin^2\theta$$
we can rewrite $I$ as follows:
$$I=k(1-\sin^2\theta)$$
$$I=k-k\sin^2\theta$$
