lim x->0 [x*cosx/sinx - 1]/(x^2)
= lim x->0 [xcosx-sinx]/(x^2*sinx)
= lim x->0 [x(1-1/2*x^2)-x +o(x^3))]/(x^3) 利用等价无穷小及cosx,sinx泰勒展开
= -1/2
lim x->0 [x*cosx/sinx - 1]/(x^2)
= lim x->0 [xcosx-sinx]/(x^2*sinx)
= lim x->0 [x(1-1/2*x^2)-x +o(x^3))]/(x^3) 利用等价无穷小及cosx,sinx泰勒展开
= -1/2