是跳跃间断点，x趋于1-的时候f（x）趋于-pi/2，当x趋于1+时，f（x）趋于pi/2
f(x) = arctan[1/(x-1)] lim(x->1+) arctan[1/(x-1)] =π/2lim(x->1-) arctan[1/(x-1)] =-π/2ans :C x=1是跳跃间断点
