#### Answer

$$\cos t=\sqrt{1-\sin^2t}$$
The equation is not an identity, which can be proved by trying $t=\pi$.

#### Work Step by Step

$$\cos t=\sqrt{1-\sin^2t}$$
To disprove this equation is an identity, we can take $t=\pi$.
At $t=\pi$, $$\cos t=\cos\pi=-1$$
and $$\sqrt{1-\sin^2t}=\sqrt{1-\sin^2\pi}=\sqrt{1-0^2}=\sqrt1=1$$
As $-1\ne1$, at $t=\pi$, $\cos t\ne\sqrt{1-\sin^2t}$. The equation, therefore, is not an identity.