Answer
$$\sin^2\theta; \ 2 \cos^2\theta; \ 2 \sin^2\theta$$
Work Step by Step
The trigonometric function $\cos{2\theta}$ can be expressed in three different forms as:
(a) $\cos{2\theta)} = \cos^2\theta-\sin^2\theta$
(b) $\cos{2\theta)} = 1-2\sin^2\theta$
(c) $\cos{2\theta)} = 2\cos^2\theta-1$
Therefore, our required missing expressions or words in the given statement are:
$$\sin^2\theta; \ 2 \cos^2\theta; \ 2 \sin^2\theta$$
