x→π/2lim (sinx-1)*(tanx)还原，t=π/2-x=lim(t→0) (sin(π/2-t)-1)*(tan(π/2-t))=lim (cost-1)*(cott)=-lim (1-cost)cost / sint利用等价无穷小：1-cosx~x^2/2，sinx~x=-lim t^2 / 2t * lim cost=0有不懂欢迎追问`~
