limx→无穷 {f(1)-f(x-1)} /2x
=limx→无穷-1/2* {f(1）-f(x-1)｝/（1-（x-1))
=-1/2*limx→无穷 {f(1)-f(x-1)} /(1-(x-1))
=-1/2*f'(1)
=-1
f'(1)=2