Answer
0
Work Step by Step
We use the addition formula for cosine, $\cos(s+t)=\cos s\cos t-\sin s\sin t$, with $s=10^\circ$ and $t=80^\circ$.
$\cos(10^\circ) \cos(80^\circ)-\sin(10^\circ)\sin(80^\circ)$
$=\cos(10^\circ+80^\circ)$
$=\cos(90^\circ)$
$=0$
