In higher level mathematics, if we know two angles of a triangle and one of the sides, we can calculate the two missing sides. The good news is we don't have to worry about that on the GRE. However, we do have to memorize two specific cases. The second is below:

$45$-$45$-$90$ Triangles

For example, if the side opposite a $45$° angle in a right triangle is $4$, we know the other side is also $4$ and the hypotenuse is $4\sqrt{2}$.

If we're given only the hypotenuse, no problem. We can still calculate the other two sides.

For example, if the hypotenuse is $7$, each other side is $\frac{7}{\sqrt{2}}$ (notice we divided by $\sqrt{2}$ here).