\(\frac{4ab}{1+ab}\le\frac{4ab}{2\sqrt{ab}}=2\sqrt{ab}\le a+b\)
Dấu "=" xảy ra khi a=b=1
\(\frac{4ab}{1+ab}\le\frac{4ab}{2\sqrt{ab}}=2\sqrt{ab}\le a+b\)
Dấu "=" xảy ra khi a=b=1