[x*x*(2-2x)]的立方根<=[x+x+(2-2x)]/3
[x*x*(2-2x)]的立方根<=2/3
所以x²(2-2x)<=(2/3)³=8/27
乘以5/2
5x²(1-x)<=(5/2)*(8/27)=20/27
所以最大值=20/27