Answer
$$0$$
Work Step by Step
Apply the sum formula:
$\cos(A+B)=\cos(A)\cos(B) -\sin(A)\sin(B)$
Therefore,
$$\cos(70^\circ) \ \cos(20^\circ) -\sin(70^\circ) \ \sin(20^\circ)\\
=\cos \ (70^\circ+20^\circ)\\
=\cos \ (90^\circ)\\
=0$$
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