x→1+时，1/(x-1)→+∞，arctan(1/(x-1)→π/2，lin(x→1+)f(x)=π/2，
x→1-时，1/(x-1)→-∞，arctan(1/(x-1)→-π/2，lin(x→1+)f(x)=-π/2.
lim(x->1-)f(x)
=lim(x->1-){arctan[1/(x-1)]
= -π/2
lim(x->1+)f(x)
=lim(x->1+){arctan[1/(x-1)]
= π/2
lim(x->1)f(x) 不存在
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