（1+t）^1/2=1+t/2+...
所以
lim(x趋向于0) [(1+tanx)^1/2-(1-sinx)^1/2] / x
=lim(x趋向于0) [1+tanx/2-1+sinx/2] / x
=lim(x趋向于0) [x/2+x/2] / x
=1