Question

Cho ba số dương 0 ≤ a≤ b ≤ c ≤ 1 CMR \(\frac{a}{bc+1}\)+ \(\frac{b}{ac+1}+\frac{c}{ab+1}\)≤ 2

 

 

Answer 1

Ta có : \(0\le a\le b\le1\)\(\Rightarrow\hept{\begin{cases}a-1\le0\\b-1\le0\end{cases}}\)

\(\Rightarrow\)\(\left(a-1\right)\left(b-1\right)\ge0\Rightarrow ab-a-b+1\ge0\)

\(\Rightarrow ab+1\ge a+b\)\(\Rightarrow\frac{1}{ab+1}\le\frac{1}{a+b}\Rightarrow\frac{c}{ab+1}\le\frac{c}{a+b}\)( vì c \(\ge\)0 )

Mà \(\frac{c}{a+b}\le\frac{2c}{a+b+c}\Rightarrow\frac{c}{ab+1}\le\frac{2c}{a+b+c}\)

tương tự : \(\frac{a}{bc+1}\le\frac{2a}{a+b+c};\frac{b}{ac+1}\le\frac{2b}{a+b+c}\)

\(\Rightarrow\frac{a}{bc+1}+\frac{b}{ac+1}+\frac{c}{ab+1}\le\frac{2\left(a+b+c\right)}{a+b+c}=2\)