Answer
$\cos A = \sin (90^{\circ} -A) = \sin B$
Work Step by Step
Use cofunctions $\cos x = \sin (90^{\circ} -x)$
In a triangle, you have :
$m(\angle B) = 90^{\circ} - m(\angle A)$
Therefore using the cofunction formula above
$\cos A = \sin (90^{\circ} -A) = \sin B$
