可以用基本不等式来做
因为x+1-x=1
所以a/x+b/(1-x)=a[x+(1-x)]/x+b[x+(1-x)]/(1-x)
=a+a*(1-x)/x+b*x/(1-x)+b
≥a+b+2√[(a*(1-x)/x)*(b*x/(1-x))]
=a+b+2√(ab)
=(√a+√b)^2