介绍两种方法：
1.洛必达法则
lim(1-x)tan[(πx)/2]
=lim(1-x)/cot[(πx)/2]
=lim(-1)/{-(π/2)[csc(πx/2)]^2}
=(2/π)lim[sin(πx/2)]^2
=(2/π)[sin(π/2)]^2
=2/π
2.等价代换
令t=1-x，则x=1-t
原式=limttan[(π(1-t))/2]
=limttan[(π/2)-(πt/2)]
=limtcot(πt/2)
=limt/tan(πt/2)
=(2/π)lim(πt/2)/tan(πt/2)
=2/π(等价无穷小代换)
