a/ \(\frac{1}{a}+\frac{1}{b}\ge\frac{4}{a+b}\) ; \(\frac{1}{b}+\frac{1}{c}\ge\frac{4}{b+c}\) ; \(\frac{1}{c}+\frac{1}{a}\ge\frac{4}{c+a}\)

Cộng theo vế :

\(2\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)\ge4\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\right)\)

\(\Leftrightarrow\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\ge2\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\right)\)

b/ \(\frac{1}{a+b}+\frac{1}{b+c}\ge\frac{4}{a+2b+c}\)

\(\frac{1}{b+c}+\frac{1}{c+a}\ge\frac{4}{b+2c+a}\)

\(\frac{1}{c+a}+\frac{1}{a+b}\ge\frac{4}{c+b+2a}\)

Cộng theo vế :

\(2\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\right)\ge4\left(\frac{1}{2a+b+c}+\frac{1}{2b+c+a}+\frac{1}{2c+a+b}\right)\)

\(\Leftrightarrow\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\ge2\left(\frac{1}{2a+b+c}+\frac{1}{a+2b+c}+\frac{1}{a+b+2c}\right)\)

d/ Đặt \(x=a+b\) , \(y=b+c\) , \(z=c+a\)

thì : \(a=\frac{x+z-y}{2}\) ; \(b=\frac{x+y-z}{2}\) ; \(c=\frac{y+z-x}{2}\)

Ta có : \(\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}=\frac{\frac{x+z-y}{2}}{y}+\frac{\frac{x+y-z}{2}}{z}+\frac{\frac{y+z-x}{2}}{x}\)

\(=\frac{z+x-y}{2y}+\frac{x+y-z}{2z}+\frac{y+z-x}{2x}=\frac{1}{2}\left(\frac{x}{y}+\frac{y}{x}+\frac{z}{y}+\frac{y}{z}+\frac{z}{x}+\frac{x}{z}-3\right)\)

\(=\frac{1}{2}\left(\frac{x}{y}+\frac{y}{x}+\frac{y}{z}+\frac{z}{y}+\frac{z}{x}+\frac{x}{z}\right)-\frac{3}{2}\ge\frac{1}{2}.6-\frac{3}{2}=\frac{3}{2}\)

b/ \(a^2\left(1+b^2\right)+b^2\left(1+c^2\right)+c^2\left(1+a^2\right)\ge6abc\)

\(\Leftrightarrow\left(a^2b^2-2abc+c^2\right)+\left(b^2c^2-2abc+a^2\right)+\left(c^2a^2-2abc+b^2\right)\ge0\)

\(\Leftrightarrow\left(ab-c\right)^2+\left(bc-a\right)^2+\left(ca-b\right)^2\ge0\) (luôn đúng)

Vậy bđt ban đầu dc chứng minh.

c/ \(\left(a+b\right)^2\ge4ab\Leftrightarrow\frac{ab}{a+b}\le\frac{a+b}{4}\)

Tương tự : \(\frac{bc}{b+c}\le\frac{b+c}{4}\) ; \(\frac{ac}{a+c}\le\frac{a+c}{4}\)

Cộng theo vế : \(\frac{ab}{a+b}+\frac{bc}{b+c}+\frac{ca}{a+c}\le\frac{2\left(a+b+c\right)}{4}=\frac{a+b+c}{2}\)

ok , cảm ơn bạn !!!

Bài toán rất hay và bổ ích !!!

Đây nhé 

Đặt b + c = x ; c + a = y ;  a + b = z 

\(\Rightarrow\hept{\begin{cases}x+y=2c+b+a=2c+z\\y+z=2a+b+c=2a+x\\x+z=2b+a+c=2b+y\end{cases}}\)

\(\Rightarrow\frac{x+y-z}{2}=c;\frac{y+z-x}{2}=a;\frac{x+z-y}{2}=b\)

Thay vào PT đã cho ở đề bài , ta có : 

\(\frac{y+z-x}{2x}+\frac{x+z-y}{2y}+\frac{x+y-z}{2z}\)

\(=\frac{1}{2}\left(\frac{y}{x}+\frac{z}{x}+\frac{x}{y}+\frac{z}{y}+\frac{x}{z}+\frac{y}{z}-3\right)\)

\(\ge\frac{1}{2}\left(2+2+2-3\right)=\frac{3}{2}\)

( cái này cô - si cho x/y + /x ; x/z + z/x ; y/z + z/y) 

e cũng có 1 vài cách chứng minh khá là cổ điển ạ !

Sử dụng BĐT AM-GM ta có :

\(\frac{a^2}{b+c}+\frac{b+c}{4}\ge2\sqrt{\frac{a^2}{b+c}.\frac{b+c}{4}}=2.\frac{a}{2}=a\)

Bằng cách chứng minh tương tự :

\(\frac{b^2}{a+c}+\frac{a+c}{4}\ge b;\frac{c^2}{a+b}+\frac{a+b}{4}\ge c\)

Cộng theo vế các bđt cùng chiều ta được :

\(\frac{a^2}{c+b}+\frac{b^2}{a+c}+\frac{c^2}{a+b}+\frac{2\left(a+b+c\right)}{4}\ge a+b+c\)

\(< =>\frac{a^2}{b+c}+\frac{a}{2}+\frac{b^2}{a+c}+\frac{b}{2}+\frac{c^2}{a+b}+\frac{c}{2}\ge a+b+c\)

\(< =>\frac{a^2}{b+c}+a+\frac{b^2}{a+c}+b+\frac{c^2}{a+b}+c\ge\frac{3}{2}\left(a+b+c\right)\)

\(< =>\frac{a\left(a+b+c\right)}{b+c}+\frac{b\left(a+b+c\right)}{a+c}+\frac{c\left(a+b+c\right)}{b+a}\ge\frac{3}{2}\left(a+b+c\right)\)

\(< =>\frac{a}{b+c}+\frac{b}{a+c}+\frac{c}{a+b}\ge\frac{3}{2}\left(Q.E.D\right)\)

Nhân cả 2 vế với a+b+c 

Chứng minh \(\frac{a}{b}+\frac{b}{a}\ge2\) tương tự với \(\frac{b}{c}+\frac{c}{b};\frac{c}{a}+\frac{a}{c}\)

\(\Leftrightarrow\frac{a}{b}+\frac{b}{a}-2\ge0\Leftrightarrow\frac{a^2-2ab+b^2}{ab}\ge0\Leftrightarrow\frac{\left(a-b\right)^2}{ab}\ge0\)luôn đúng do a;b>0

dễ rồi nhé

b) \(P=\frac{x}{x+1}+\frac{y}{y+1}+\frac{z}{z+1}\)

\(P=\left(\frac{x+1}{x+1}+\frac{y+1}{y+1}+\frac{z+1}{z+1}\right)-\left(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\right)\)

\(P=\left(1+1+1\right)-\left(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\right)\)

\(P=3-\left(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\right)\)

Áp dụng bđt Cauchy Schwarz dạng Engel (mình nói bđt như vậy,chỗ này bạn cứ nói theo cái bđt đề bài cho đi) ta được: 

\(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\ge\frac{\left(1+1+1\right)^2}{x+1+y+1+z+1}=\frac{9}{4}\)

=>\(P=3-\left(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\right)\le3-\frac{9}{4}=\frac{3}{4}\)

=>P_max=3/4 <=> x=y=z=1/3

c) Áp dụng bđt Cauchy Schwarz dạng Engel ta được:

\(P=\frac{1}{a^2+2bc}+\frac{1}{b^2+2ac}+\frac{1}{c^2+2ab}\ge\frac{\left(1+1+1\right)^2}{a^2+2ab+b^2+2ac+c^2+2ab}=\frac{9}{\left(a+b+c\right)^2}\)

<=>\(P\ge\frac{9}{\left(a+b+c\right)^2}\ge\frac{9}{1^2}=9\)

Vậy P_min=9 <=> a=b=c=1/3

Ta có:

\(\left(a+b+c\right)\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\right)=\frac{1}{2}\left(\left(a+b\right)+\left(b+c\right)+\left(c+a\right)\right)\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\right)\)

\(\ge\frac{1}{2}.3\sqrt[3]{\left(a+b\right)\left(b+c\right)\left(c+a\right)}.3\sqrt[3]{\frac{1}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}}=\frac{9}{2}\)

BĐT trên \(=\frac{9}{2}\). Còn cách làm thì giống bạn alibaba nguyễn .

~~~ Chúc bạn học giỏi ~~~

Giải:

Áp dụng BĐT Cauchy - Schwarz dạng Engel ta có:

\(\left(a+b+c\right)\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{a+c}\right)\)

\(\ge\left(a+b+c\right).\frac{\left(1+1+1\right)^2}{2\left(a+b+c\right)}\ge\frac{9}{2}\)

Vậy \(\left(a+b+c\right)\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{a+c}\right)\ge\frac{9}{2}\) (Đpcm)

a)Áp dụng Bđt Cô si ta có:

\(\frac{1}{a+1}+\frac{1}{b+1}+\frac{1}{c+1}\ge\frac{3}{\sqrt[3]{\left(a+1\right)\left(b+1\right)\left(c+1\right)}}\)

\(\frac{a}{a+1}+\frac{b}{b+1}+\frac{c}{c+1}\ge\frac{3\sqrt[3]{abc}}{\sqrt[3]{\left(a+1\right)\left(b+1\right)\left(c+1\right)}}\)

Cộng theo vế 2 bđt trên ta có:

\(3\ge\frac{3\left(\sqrt[3]{abc}+1\right)}{\sqrt[3]{\left(1+a\right)\left(1+b\right)\left(1+c\right)}}\)\(\Rightarrow\left(1+a\right)\left(1+b\right)\left(1+c\right)\ge\left(1+\sqrt[3]{abc}\right)^3\)

Dấu = khi a=b=c

b)Áp dụng Bđt Cô-si ta có:

\(\frac{bc}{a}+\frac{ca}{b}\ge2\sqrt{\frac{bc^2a}{ab}}=2c\)

\(\frac{ca}{b}+\frac{ab}{c}\ge2\sqrt{\frac{ca^2b}{bc}}=2a\)

\(\frac{bc}{a}+\frac{ab}{c}\ge2\sqrt{\frac{b^2ac}{ac}}=2b\)

Cộng theo vế 3 bđt trên ta có:

\(2\left(\frac{bc}{a}+\frac{ca}{b}+\frac{ab}{c}\right)\ge2\left(a+b+c\right)\)

\(\Rightarrow\frac{bc}{a}+\frac{ca}{b}+\frac{ab}{c}\ge a+b+c\)

Đấu = khí a=b=c

bn sử đấu = khí là dấu = khi nhé

2) Theo nguyên lí Dirichlet, trong ba số \(a^2-1;b^2-1;c^2-1\) có ít nhất hai số nằm cùng phía với 1.

Giả sử đó là a² - 1 và b² - 1. Khi đó \(\left(a^2-1\right)\left(b^2-1\right)\ge0\Leftrightarrow a^2b^2-a^2-b^2+1\ge0\)

\(\Rightarrow a^2b^2+3a^2+3b^2+9\ge4a^2+4b^2+8\)

\(\Rightarrow\left(a^2+3\right)\left(b^2+3\right)\ge4\left(a^2+b^2+2\right)\)

\(\Rightarrow\left(a^2+3\right)\left(b^2+3\right)\left(c^2+3\right)\ge4\left(a^2+b^2+1+1\right)\left(1+1+c^2+1\right)\) (2)

Mà \(4\left[\left(a^2+b^2+1+1\right)\left(1+1+c^2+1\right)\right]\ge4\left(a+b+c+1\right)^2\) (3)(Áp dụng Bunhicopxki và cái ngoặc vuông)

Từ (2) và (3) ta có đpcm.

Sai thì chịu

Xí quên bài 2 b:v

b) Không mất tính tổng quát, giả sử \(\left(a^2-\frac{1}{4}\right)\left(b^2-\frac{1}{4}\right)\ge0\)

Suy ra \(a^2b^2-\frac{1}{4}a^2-\frac{1}{4}b^2+\frac{1}{16}\ge0\)

\(\Rightarrow a^2b^2+a^2+b^2+1\ge\frac{5}{4}a^2+\frac{5}{4}b^2+\frac{15}{16}\)

Hay \(\left(a^2+1\right)\left(b^2+1\right)\ge\frac{5}{4}\left(a^2+b^2+\frac{3}{4}\right)\)

Suy ra \(\left(a^2+1\right)\left(b^2+1\right)\left(c^2+1\right)\ge\frac{5}{4}\left(a^2+b^2+\frac{1}{4}+\frac{1}{2}\right)\left(\frac{1}{4}+\frac{1}{4}+c^2+\frac{1}{2}\right)\)

\(\ge\frac{5}{4}\left(\frac{1}{2}a+\frac{1}{2}b+\frac{1}{2}c+\frac{1}{2}\right)^2=\frac{5}{16}\left(a+b+c+1\right)^2\) (Bunhiacopxki) (đpcm)

Đẳng thức xảy ra khi \(a=b=c=\frac{1}{2}\)

Cách nữa cho bài 2:

2a) Ta có: \(4\left(a^2+1+2\right)\left(1+1+\frac{\left(b+c\right)^2}{2}\right)\ge4\left(a+b+c+1\right)^2\)

Hay \(4\left(a^2+3\right)\left(2+\frac{\left(b+c\right)^2}{2}\right)\ge4\left(a+b+c+1\right)^2=VP\)

Như vậy ta quy bài toán về chứng minh: \(\left(b^2+3\right)\left(c^2+3\right)\ge4\left(2+\frac{\left(b+c\right)^2}{2}\right)\)

\(\Leftrightarrow b^2c^2+b^2+c^2+1\ge4bc\Leftrightarrow\left(bc-1\right)^2+\left(b-c\right)^2\ge0\)(đúng)

Đẳng thức xảy ra khi a = b = c = 1

b) Áp dụng BĐT Bunhiacopxki:\(\left(a^2+\frac{1}{4}+\frac{1}{4}+\frac{1}{2}\right)\left(\frac{1}{4}+b^2+c^2+\frac{1}{2}\right)\ge\frac{1}{4}\left(a+b+c+1\right)^2\)

\(\Rightarrow\frac{5}{4}\left(a^2+1\right)\left(b^2+c^2+\frac{3}{4}\right)\ge\frac{5}{16}\left(a+b+c+1\right)^2\)

Từ đó ta có thể quy bài toán về chứng minh: \(\left(b^2+1\right)\left(c^2+1\right)\ge\frac{5}{4}\left(b^2+c^2+\frac{3}{4}\right)\)

...

Bài 3:Sửa đề a, b, c >0

Có:  \(\frac{a^3}{b^2}+\frac{a^3}{b^2}+b\ge3\sqrt[3]{\frac{a^6}{b^3}}=\frac{3a^2}{b}\)

Tương tự: \(\frac{2b^3}{c^2}+c\ge\frac{3b^2}{c};\frac{2c^3}{a^2}+a\ge\frac{3c^2}{a}\)

Cộng theo vế 3 BĐT trên: \(2\left(\frac{a^3}{b^2}+\frac{b^3}{c^2}+\frac{c^3}{a^2}\right)+a+b+c\ge3\left(\frac{a^2}{b}+\frac{b^2}{c}+\frac{c^2}{a}\right)\)

\(=2\left(\frac{a^2}{b}+\frac{b^2}{c}+\frac{c^2}{a}\right)+\left(\frac{a^2}{b}+\frac{b^2}{c}+\frac{c^2}{a}\right)\)

\(\ge2\left(\frac{a^2}{b}+\frac{b^2}{c}+\frac{c^2}{a}\right)+a+b+c\)

Từ đó ta có đpcm.