属于0*∞型,变形后用罗比塔法则：
lim(x-->π/2)（sinx-1)tanx
=lim(x-->π/2)（sinx-1)/cotx
=lim(x-->π/2)cosx/[-csc^2(x)]
=lim(x-->π/2)[-sin^(x)cosx]
=0