Cho a,b,c$>$0.Chứng minh rằng:
$(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})(\frac{1}{1+a}+\frac{1}{1+b}+\frac{1}{1+c})\geq \frac{9}{a+b+c}$
Cho a,b,c$>$0.Chứng minh rằng:
$(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})(\frac{1}{1+a}+\frac{1}{1+b}+\frac{1}{1+c})\geq \frac{9}{a+b+c}$
ONG NGỰA 97.
Cho a,b,c$>$0.Chứng minh rằng:
$(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})(\frac{1}{1+a}+\frac{1}{1+b}+\frac{1}{1+c})\geq \frac{9}{a+b+c}$
Hình như đề bài sai. Cho a=b=c=3 thì VT<VP
"The first analogy that came to my mind is of immersing the nut in some softening liquid, and why not simply water? From time to time you rub so the liquid penetrates better, and otherwise you let time pass. The shell becomes more flexible through weeks and months—when the time is ripe, hand pressure is enough, the shell opens like a perfectly ripened avocado!" - Grothendieck
Mình xin lỗi, mình gõ lộn đề, đề đúng là: cho a,b,c dương. CMR
$(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})(\frac{1}{1+a}+\frac{1}{1+b}+\frac{1}{1+c})\geq \frac{9}{1+abc}$
ONG NGỰA 97.
Mình xin lỗi, mình gõ lộn đề, đề đúng là: cho a,b,c dương. CMR
$(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})(\frac{1}{1+a}+\frac{1}{1+b}+\frac{1}{1+c})\geq \frac{9}{1+abc}$
Áp dụng BĐT Hoán vị, ta có $\frac{1}{a(1+a)}+\frac{1}{b(1+b)}+\frac{1}{c(1+c)}\geq \frac{1}{b(1+c)}+\frac{1}{c(1+a)}+\frac{1}{a(1+b)}$. Do đó ta chỉ cần cm 2 BĐT sau:$\frac{1}{b(1+c)}+\frac{1}{c(1+a)}+\frac{1}{a(1+b)}\geq \frac{3}{1+abc}$ (1)
$\frac{1}{a(1+c)}+\frac{1}{b(1+a)}+\frac{1}{c(1+b)}\geq \frac{3}{1+abc}$ (2)
Chứng minh BĐT (1):Đặt $a=\frac{kx}{y}; b=\frac{ky}{z}; c=\frac{kz}{x}$. BĐT trở thành $\frac{y}{z+kx}+\frac{z}{x+ky}+\frac{x}{y+kz}\geq \frac{3k}{1+k^3}$. Áp dụng BĐT Cauchy-Schwarz, ta có VT$\geq \frac{(x+y+z)^2}{(1+k)(ab+bc+ca)}\geq \frac{3}{1+k}$. Do đó ta chỉ cần chứng minh $\frac{3}{1+k}\geq \frac{3k}{1+k^3}\Leftrightarrow (k-1)^2(k+1)\geq 0$(hiển nhiên đúng). BĐT (2) chứng minh tương tự suy ra đpcm
"The first analogy that came to my mind is of immersing the nut in some softening liquid, and why not simply water? From time to time you rub so the liquid penetrates better, and otherwise you let time pass. The shell becomes more flexible through weeks and months—when the time is ripe, hand pressure is enough, the shell opens like a perfectly ripened avocado!" - Grothendieck
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