(a+1/a)(b+1/b) =ab+a/b+b/a+1/ab =(ab+1/ab)+(a/b+b/a) >=2*根号ab*1/ab+2*根号a/b*b/a >=2+2 >=4 等号成立的条件是ab=1/ab且a/b=b/a 即a=b=+-1
