x^3+o(x^3)=x^3(1+o(1))
从而（x^3(1+o(1))）^2/3=x^2(1+o(1))^2/3=x^2(1+2/3*o(1))=x^2(1+o(1))=x^2+o(x^2).
这里省略了系数1/6,可自行加上.