f'(x)的定义是lim(h→0) [f(x+h)-f(x)]/h =f'(x)所以lim (f(x+2h)-f(x-3h))/h=lim [(f(x+2h)-f(x))+(f(x)-f(x-3h))]/h=lim [f(x+2h)-f(x)]/2h*2+[f(x)-f(x-3h)]/(3h)*(3)=2f'(x)+3f'(x)=5f'(x)