**Problem Statement**

In the following image, I cannot understand why taking the derivative of 
$\sqrt{1-y^2}$ that you must do $\frac{\frac{d}{dx}[1-y^2]}{2\sqrt{1-y^2}}$ in order to get to the solution.

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**Here are the steps (partially)**

The step that I do not understand is the one circled in red.

[![enter image description here](https://i.stack.imgur.com/ubKR0.png)](https://i.stack.imgur.com/ubKR0.png)

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**Conclusion**

As you saw, the step circled in red is the part I do not understand. To me, it makes no sense how you get to that step. It would be nice if someone could explain how to get to that step from the previous one.