ad=bc
d=bc/a
又∵a>b>0 a>c>0
∴b>d c>d
∴d最小
a=bc/d
a+d-c-b
=bc/d+d-c-b
=(bc+d^2-cd-bd)/d
=(b-d)(c-d)/d
∵d为最小
∴b-d>0 c-d>0
∴ (b-d)(c-d)/d>0
∴a+d-c-b>0
∴a+d>b+c