\begin{align} f'(x) &= - \frac{x}{\sqrt{1-x^2}} + \frac{1}{\sqrt{1-x^2}} \\ &= \frac{1-x}{\sqrt{1-x^2}} \\ &= \sqrt{\frac{1-x}{1+x}} \end{align}
\begin{align} f'(1) = 0 \end{align}
\begin{align} I &= \int_0^1 dx \int_{x^2}^x dy \left( x^2 + y^2 \right) \\ &= \int_0^1 dx \left[ x^2 y + \frac{y^3}{3} \right]_{y=x^2}^x \\ &= \int_0^1 dx \left( - \frac{1}{3} x^6 - x^4 + \frac{4}{3} x^3 \right) \\ &= \left[ - \frac{1}{21} x^7 - \frac{1}{5} x^5 + \frac{1}{3} x^4 \right]_0^1 \\ &= \frac{3}{35} \end{align}