x+y
=1*(x+y)
=(a/x+b/y)(x+y)
=a+b+(ax/y+by/x)
[均值不等式]
≥a+b+2√(ax/y*by/x)
=a+b+2√(ab)
=(√a+√b)^2
则x+y的最小值为(√a+√b)^2