Answer
a) Sine is cosine off by 90 degrees
b) .8746
Work Step by Step
a) Sine and cosine are off by 90 degrees. We can prove using a right triangle:
$$ 90-\alpha = \beta \\ cos\alpha = \frac{b}{c} \\ sin \beta=\frac{b}{c}$$
Thus:
$$ cos\alpha = sin (90-\alpha)$$
b) Plugging into the calculator, we find that both expressions equal .8746.
