+816 The function $f(x),$ defined for $x \ge 0,$ has the following properties:
$f(x) \ge \sqrt{x}$ for all $x \ge 0.$
The function $f(x)$ is increasing.
The area between the graph of $y = f(x)$ for $0 \le x \le a$ and the graph of $y = x^2$ is equal to the area between the same part of the graph and the $y$-axis. (In other words, the red area is equal to the blue area.)
(a) Find a differential equation that the function $f(x)$ satisfies. (In particular, this equation will involve $f(x)$ and $f'(x).$)
(b) Prove that $f(x) = 2x^2 + kx$ for some constant $k.$
