因为当x趋近于0时，x和arctanx是等价无穷小
x->0arctanx = x-(1/3)x^3 +o(x^3)arctanx/x = 1-(1/3)x^2 +o(x^2)lim(x->0) ( arctanx/x) ^(1/x^2)=lim(x->0) [ 1- (1/3)x^2 ] ^(1/x^2)=e^(-1/3)
