Answer
$\csc$ needs to be filled in the blank.
Work Step by Step
$$\cos70^\circ=\frac{1}{A20^\circ}\hspace{1cm}(1)$$
From the Cofunction Identity:
$$\cos\theta=\sin(90^\circ-\theta)$$
So $$\cos70^\circ=\sin(90^\circ-70^\circ)=\sin20^\circ$$
Also, from Reciprocal Identity:
$$\sin\theta=\frac{1}{\csc\theta}$$
Therefore, $$\cos70^\circ=\frac{1}{\csc20^\circ}$$
We thus need to fill in the blank with $\csc$.
