f(x)=∫f'(t)dt+f(0) (∫f'(t)dt表示f'(t)在[0,x]上的积分)
则f(x)=∫f'(t)dt+f(0)≥∫kdt+f(0) = kx + f(0)
则x→+∞,f(x)→+∞,又f(0)