$\displaystyle f $ is a differentiable function at $\displaystyle [a,\infty)$.

i need to prove that:

if there is a constant $\displaystyle m>0$ which maintains that $\displaystyle f'(x)\geq m $ to every $\displaystyle x\in[a,\infty),$

so$\displaystyle lim_{x\rightarrow\infty}f(x)=\infty $... thx!!