求lim(1-x)tanπx/2 x→1
问题描述：
求lim(1-x)tanπx/2 x→1
答
lim(x→1)(1-x)tan(πx/2)
=lim(x→1)(1-x)/cot(πx/2)
=2/πlim(x→1)(-1)/-csc^2(πx/2)
=2/π