Question

若a∈ (0,＋∞) ,b∈ (0,＋∞),则有a+b≥2根号ab成立,现在已知a>0 ,b>0 ,c>0 ,d>0

Answer 1

（(b/a) +(d/c)）（(c/d)+(a/b)）

=1+1+(ad)/(bc)+(bc)/(ad)

≥2+2√[(ad)/(bc)*(bc)/(ad)]

=2+2=4

∴（(b/a) +(d/c)）（(c/d)+(a/b)）≥4