根据同阶无穷小,x→0时,sinx～x lim(x→0)cos(sinx)-cosx/x^4 =lim(x→0)cosx-cosx/x^4 =lim(x→0)cosx(1-1/x^4) =lim(x→0)cosx*lim(x→0)(1-1/x^4) =1*lim(x→0)(1-1/x^4) =-∞
