原式=lim(x->π/2)(sinx/cosx)/(sin3x/cos3x)=-lim(x->π/2)(cos3x/cosx)=-lim(x->π/2)(-3sin3x/(-sinx))=-3sin3π/2/sinπ/2=-3×（-1）/1=3
上下求导，用洛必达法则
答案是-1/9?
