a2/x + b2/(1-x)≥2|ab|/√x(1-x)
求a2/x + b2/(1-x)的最小值就是求分母的最大值
x-x^2=-(x-1/2)^2+1/4
当x=1/2时分母取最大值1/2
a2/x + b2/(1-x)的最小值=4|ab|