Question

Cho a,b,c >0 thỏa mãn a+b+c=3. Chứng minh rằng \(\frac{a+1}{b^2+1}+\frac{b+1}{c^2+1}+\frac{c+1}{a^2+1}\ge3\)

Answer 1

Ta có: \(a+1-\frac{a+1}{b^2+1}=\frac{ab^2+b^2}{b^2+1}\le\frac{ab^2+b^2}{2b}=\frac{ab}{2}+\frac{b}{2}\) vì \(b^2+1\ge2b\)

nên \(\frac{a+1}{b^2+1}\ge a+1-\frac{b}{2}-\frac{ab}{2}\) Tương tự: 

Vậy ta có: \(VT\ge a+b+c+3-\frac{a+b+c}{2}-\frac{1}{2}\left(ab+bc+ca\right)\)

Vì \(ab+bc+ca\le\frac{\left(a+b+c\right)^2}{3}=\frac{9}{3}=3\)

nên \(VT\ge3+\frac{a+b+c}{2}-\frac{1}{2}3=3+\frac{3}{2}-\frac{3}{2}=3=VP\)