$\noindent For the upper half of a circle of radius $a$ ($a$ some constant greater than $0$), we can write it as $y=\sqrt{a^2-x^2}$, $-a<x<a$. Then $y^\prime = \frac{-x}{\sqrt{a^2-x^2}}$, for $-a<x<a$. You can do a similar computation for the lower half, where $y=-\sqrt{a^2-x^2}$ $\Big{(}$well...