(a) Let $f$ and $g$ be functions from $\mathbb{R}$ to $\mathbb{R}$ and suppose that both $f$ and $g$ are differentiable at the real number $x$. Prove that the product $f g$ is also differentiable at $x$.

(b) Let $f$ be a continuous function from $\mathbb{R}$ to $\mathbb{R}$ and let $g(x)=x^{2} f(x)$ for every $x$. Prove that $g$ is differentiable at $x$ if and only if either $x=0$ or $f$ is differentiable at $x$.

(c) Now let $f$ be any continuous function from $\mathbb{R}$ to $\mathbb{R}$ and let $g(x)=f(x)^{2}$ for every $x$. Prove that $g$ is differentiable at $x$ if and only if at least one of the following two possibilities occurs:

(i) $f$ is differentiable at $x$;

(ii) $f(x)=0$ and

$$\frac{f(x+h)}{|h|^{1 / 2}} \longrightarrow 0 \quad \text { as } \quad h \rightarrow 0$$