## Trigonometry (11th Edition) Clone

$\cos A = \sin (90^{\circ} -A) = \sin B$
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 $\cos A = \sin (90^{\circ} -A) = \sin B$