跟刚才那题一样的啊
x→0时,1-cosx等价于x^2/2
lim(x→0)(1-cosx)^2/x^3
=lim(x→0)(x^4/4)/x^3
=lim(x→0)x/4=0
所以（1-cosx）^2是x^3的高阶无穷小