# How do you evaluate cos[2arcsin(7/25)]?

$\frac{527}{625} = \frac{17 \cdot 31}{5} ^ 4$
if $\sin \alpha = \frac{7}{25}$ then how much is $x = \cos 2 \alpha$ ?
$x = {\cos}^{2} \alpha - {\sin}^{2} \alpha = 1 - 2 {\sin}^{2} \alpha = 1 - \frac{2 \cdot 49}{625}$
$x = \frac{625 - 98}{625}$