Question

已知a,b,c,d为正实数,求证（ad+bc）/bd+(bc+ad)/ac≥4

Answer 1

（ad+bc）/bd+(bc+ad)/ac

=a/b+c/d+b/a+d/c

=(a/b+b/a)+(c/d+d/c)

>=2√(a/b*b/a)+2√(c/d*d/c)

=4