Using the definition of the derivative to prove a constant function – math.stackexchange.com

Using the definition of the derivative to prove a constant function –
math.stackexchange.com

I am presented with the following task: "Let $f$ be a function defined on
the interval $I$. All we know about $f$ is that there is a constant $K$
such that $$|f(a) - f(b)| \leq K|a-b|^2$$ for all …