Solving equation with fraction I don't understand how to get from the second to the third step in this equation:
$  - \frac { \sqrt { 2 - x ^ { 2 } } - x \left( \frac { - x } { \sqrt { 2 - x ^ { 2 } } } \right) } { \left( \sqrt { 2 - x ^ { 2 } } \right) ^ { 2 } } = - \frac { \sqrt { 2 - x ^ { 2 } } + \frac { x ^ { 2 } } { \sqrt { 2 - x ^ { 2 } } } } { \left( \sqrt { 2 - x ^ { 2 } } \right) ^ { 2 } } = - \frac { \frac { 2 - x ^ { 2 } } { \sqrt { 2 - x ^ { 2 } } } + \frac { x ^ { 2 } } { \sqrt { 2 - x ^ { 2 } } } } { \left( \sqrt { 2 - x ^ { 2 } } \right) ^ { 2 } } = - \frac { 2 } { \left( \sqrt { 2 - x ^ { 2 } } \right) ^ { 3 } } $
Why can we just add $ 2 - x ^ { 2 } $ in the numerator?
Step 1 to step 2, as well as step 3 to step 4 is clear to me.

You multiply by $1$ in a fancy fashion:
$$\sqrt{2-x^2}=\sqrt{2-x^2}\cdot 1=\sqrt{2-x^2}\cdot \frac{\sqrt{2-x^2}}{\sqrt{2-x^2}}=\frac{(\sqrt{2-x^2})^2}{\sqrt{2-x^2}}=\frac{2-x^2}{\sqrt{2-x^2}}$$
This of course only holds if $2-x^2\neq 0$, as otherwise also $\sqrt{2-x^2}=0$ and the last expression(and the intermediate ones) would be ill-defined.

Note, that his holds for any $a\in (0,\infty)$ by the same method as above, i.e.
$$\sqrt{a}=\sqrt{a}\cdot 1=\sqrt{a}\cdot\frac{\sqrt{a}}{\sqrt{a}}=\frac{(\sqrt{a})^2}{\sqrt{a}}=\frac{a}{\sqrt{a}}$$
