若xy∈R+ x+y=2 求证1/x+1/y≥2
问题描述：
若xy∈R+ x+y=2 求证1/x+1/y≥2
答
(x+y)^2-(x-y)^2=4xy
=>
4>=4xy
=>
xy=>
(x+y)/xy>=2
=>
1/x+1/y>=2肿么办 我看不懂这一步(x+y)^2-(x-y)^2=4xy=>4>=4xy(x+y)^2=4xy+(x-y)^2=>(x+y)^2>=4xy代条件=>4>=4xy