cosx-1=1-2(sinx/2)^2-1=-2(sinx/2)^2
而-sin(x/2)^2和-(x/2)^2=-x^2/4是等价无穷小
因为(1+x)^y-1和yx是等价无穷小
所以(1+ax^2)^1/3-1和1/3ax^2是等价无穷小
由题意则-1/4=1/3a a=-3/4