CPM Homework Help : PC3 Problem 8-74

Given $s^{2} + c^{2} = 1$ , rewrite the following expressions using only $c$ .

$c^2-s^2$
Hint (a):
Rewrite the given equation for $s^{2}$ .
Then substitute in that expression.

$\frac{c}{1+s}+\frac{c}{1-s}$
Step (b):
$\frac{c}{1+s}\cdot\frac{1-s}{1-s}+\frac{c}{1-s}\cdot\frac{1+s}{1+s}$