#### Answer

$$\cos210^\circ=\cos150^\circ\cos60^\circ-\sin150^\circ\sin60^\circ$$
17 is matched with I.

#### Work Step by Step

$$\cos210^\circ$$
We can write $210^\circ$ as the sum of $150^\circ$ and $60^\circ$.
Thus, $$\cos210^\circ=\cos(150^\circ+60^\circ)$$
Here we can apply the sum identity for cosine for $\cos(150^\circ+60^\circ)$.
$$\cos(A+B)=\cos A\cos B-\sin A\sin B$$
So, if we replace $A=150^\circ$ and $B=60^\circ$, we would have
$$\cos210^\circ=\cos150^\circ\cos60^\circ-\sin150^\circ\sin60^\circ$$
This fits with choice I. Therefore, we should match 17 with I.