3/cho a,b,c >0 và $6(\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2})\leq 1+\frac{1}{a}+\frac{1}{b}+\frac{1}{c}$ CMR
$\sum \frac{1}{10a+b+c}\leq \frac{1}{12}$
3,Ta có bđt cần cm <=> $\sum \frac{144}{10a+b+c}\leq 12$
Mà$\frac{144}{10a+b+c}\leq \frac{10}{a}+\frac{1}{b}+\frac{1}{c}$
=>Ta chỉ cần c/m $\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\leq 1$
Đặt $\sum \frac{1}{a}=x$ ta có $2x^{2}\leq 6(\sum \frac{1}{a^{2}})\leq 1+x$
<=> $(x-1)(2x+1)\leq 0<=> x\leq 1$ =>ĐPCM