lim(1-x)*tan(∏x/2)=lim [(1-x)*sin(∏x/2)]/cos(∏x/2)
用罗比达法则得
lim[-sin(∏x/2)+(∏/2)*(1-x)*cos(∏x/2)]/(-∏/2)*sin(∏x/2)
=(-1+0)/(-∏/2)
=2/∏