Prove that $f(x)=x^2$ decreases on the interval $(-\infty,0]$ and increases on the interval $[0,\infty)$.

Problem: 

Prove that $f(x)=x^2$ decreases on the interval $(-\infty,0]$ and increases on the interval $[0,\infty)$.

Answer: 

It is true that $f(x)=x^2$ decreases on the interval $(-\infty,0]$ and increases on the interval $[0,\infty)$.