1) Tìm GTNN : 

Ta có : \(\frac{x}{y+1}+\frac{y}{x+1}=\frac{x^2}{xy+x}+\frac{y^2}{xy+y}\ge\frac{\left(x+y\right)^2}{2xy+\left(x+y\right)}\ge\frac{1}{\frac{\left(x+y\right)^2}{2}+1}=\frac{1}{\frac{1}{2}+1}=\frac{2}{3}\)

Dấu "=" xảy ra \(\Leftrightarrow x=y=\frac{1}{2}\)

2) Áp dụng BĐT Svacxo ta có :

\(\frac{a^2}{1+b}+\frac{b^2}{1+c}+\frac{c^2}{1+a}\ge\frac{\left(a+b+c\right)^2}{3+a+b+c}=\frac{9}{6}=\frac{3}{2}\)

Dấu "=" xảy ra \(\Leftrightarrow a=b=c=1\)

2/ Áp dụng bđt Cô- si cho 2 số dương ta có :

\(\frac{a^2}{1+b}+\frac{1+b}{4}\ge2\sqrt{\frac{a^2}{1+b}\frac{1+b}{4}}=a\)

Tương tự ta có \(\frac{b^2}{1+c}+\frac{1+c}{4}\ge b;\frac{c^2}{1+a}+\frac{1+a}{4}\ge c\)

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

\(\Rightarrow\frac{a^2}{1+b}+\frac{b^2}{1+c}+\frac{c^2}{1+a}\ge3-\frac{1}{4}\left(a+b+c\right)-\frac{3}{4}=3-\frac{1}{4}.3-\frac{3}{4}=\frac{3}{2}\)

Dấu "=" xảy ra <=> a=b=c=1 

Ta chứng minh BĐT sau với các số dương:

\(\dfrac{1}{x}+\dfrac{1}{y}\ge\dfrac{4}{x+y}\)

Thật vậy, BĐT tương đương: \(\dfrac{x+y}{xy}\ge\dfrac{4}{x+y}\Leftrightarrow\left(x+y\right)^2\ge4xy\)

\(\Leftrightarrow x^2-2xy+y^2\ge0\Leftrightarrow\left(x-y\right)^2\ge0\) (luôn đúng)

Áp dụng:

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

Cộng vế với vế:

\(2\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)\ge\dfrac{4}{a+b}+\dfrac{4}{b+c}+\dfrac{4}{c+a}\)

\(\Leftrightarrow\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\ge\dfrac{2}{a+b}+\dfrac{2}{b+c}+\dfrac{2}{c+a}\)

b.

Ta có:

\(\dfrac{1}{a}+\dfrac{1}{b}\ge\dfrac{4}{a+b}\Rightarrow\dfrac{3}{a}+\dfrac{3}{b}\ge\dfrac{12}{a+b}\) (1)

\(\dfrac{1}{b}+\dfrac{1}{c}\ge\dfrac{4}{b+c}\Rightarrow\dfrac{2}{b}+\dfrac{2}{c}\ge\dfrac{8}{b+c}\) (2)

\(\dfrac{1}{c}+\dfrac{1}{a}\ge\dfrac{4}{c+a}\) (3)

Cộng vế với vế (1); (2) và (3):

\(\dfrac{4}{a}+\dfrac{5}{b}+\dfrac{3}{c}\ge4\left(\dfrac{3}{a+b}+\dfrac{2}{b+c}+\dfrac{1}{c+a}\right)\) (đpcm)

Dấu "=" xảy ra khi \(a=b=c\)

Sr nha,giờ ms đọc dc tin nhắn :(

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

Dấu "=" xảy ra tại \(a=b=\frac{1}{2}\)

Svacc -xơ

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

(Dấu "="\(\Leftrightarrow a=b=\frac{1}{2}\))

ta có

\(\frac{a}{1+b^2}=a-\frac{ab^2}{1+b^2}\ge a-\frac{ab^2}{2b}=a-\frac{ab}{2}\left(AM-GM\right)\)

tương tự ta có

\(\frac{a}{1+b^2}+\frac{b}{1+c^2}+\frac{c}{1+a^2}\ge\left(a+b\ge+c\right)-\frac{1}{2}\left(ab+bc+ca\right)\ge\frac{3}{2}\)

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

\(\frac{a}{b^2}+\frac{1}{a}\ge\frac{2}{b}\) BĐT Cô-si

Tương tự suy ra đpcm

\(\frac{a}{1+b^2}=a-\frac{ab^2}{1+b^2}\ge a-\frac{ab^2}{2b}=a-\frac{ab}{2}\) (Cô si ngược + Rút gọn)

Tương tự \(\frac{b}{1+c^2}\ge b-\frac{bc}{2};\frac{c}{1+a^2}\ge c-\frac{ca}{2}\)

Cộng theo vế 3 BĐT,ta được: \(VT\ge\left(a+b+c\right)-\left(\frac{ab+bc+ca}{2}\right)=3-\frac{ab+bc+ca}{2}\)

Mặt khác,ta có BĐT \(xy+yz+zx\le\frac{\left(x+y+z\right)^2}{3}\) (bạn tự c/m,không làm được ib)

Thay x = a; y = b ; z = c,ta có: \(ab+bc+ca\le\frac{\left(a+b+c\right)^2}{3}=\frac{9}{3}=3\)

Suy ra\(VT\ge3-\frac{ab+bc+ca}{2}\ge3-\frac{3}{2}=\frac{3}{2}^{\left(đpcm\right)}\)

Dấu "=" xảy ra khi a = b = c = 1

\(A=\frac{2ab}{4ab}+\frac{2ab}{a^2+4b^2}+\frac{1}{8ab}-\frac{1}{2}\)

áp dụng bđt AM-GM , a,b> 0

\(\Rightarrow A\ge2ab\left(\frac{4}{4ab+a^2+4b^2}\right)+\frac{1}{8ab}-\frac{1}{2}\)

\(\Rightarrow A\ge\frac{8ab}{1}+\frac{1}{8ab}-\frac{1}{2}\)

\(\Rightarrow A\ge2-\frac{1}{2}=\frac{3}{2}\)