\(\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\ge\frac{3}{2}\)

<=> \(\frac{a+b+c}{b+c}+\frac{a+b+c}{c+a}+\frac{a+b+c}{a+b}\ge\frac{9}{2}\)

<=> \(2\left(a+b+c\right)\left(\frac{1}{b+c}+\frac{1}{c+a}+\frac{1}{a+b}\right)\ge9\)

<=> \(\left(a+b+b+c+c+a\right)\left(\frac{1}{b+c}+\frac{1}{c+a}+\frac{1}{a+b}\right)\ge9\)

<=> \(\frac{a+b}{b+c}+\frac{a+b}{c+a}+1+1+\frac{b+c}{c+a}+\frac{b+c}{a+b}+\frac{c+a}{b+c}+1+\frac{c+a}{a+b}\ge9\)

<=> \(\left(\frac{a+b}{b+c}+\frac{b+c}{a+b}\right)+\left(\frac{a+b}{c+a}+\frac{c+a}{a+b}\right)+\left(\frac{b+c}{c+a}+\frac{c+a}{b+c}\right)\ge6\)(đúng)

=> ĐPCM

Mình làm cách đơn giản nhất nhá :))

Ta có:

\(\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}+3=\left(\frac{a}{b+c}+1\right)+\left(\frac{b}{c+a}+1\right)+\left(\frac{c}{a+b}+1\right)\)

\(=\left(a+b+c\right)\left(\frac{1}{b+c}+\frac{1}{c+a}+\frac{1}{a+b}\right)\ge\frac{9\left(a+b+c\right)}{2\left(a+b+c\right)}=\frac{9}{2}\left(Cauchy-Schwarz\right)\)

Hay \(\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}+3\ge\frac{9}{2}\Leftrightarrow\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\ge\frac{3}{2}\)

a) O là trung điểm CE

b) DHF = DCF = DCA = DKA. (hai goc đồng vị)

c) Tứ giác ABCH nội tiếp nên H di chuyển trên đường tròn ngoại tiếp của tam giác ABC

GTNN là 2/3 khi x = 1

Đây là câu bđt của chuyên Quảng Nam vừa thi mà:vvv

Ta có: \(xy+yz+zx=xyz\Leftrightarrow\frac{1}{x}+\frac{1}{y}+\frac{1}{z}=1\)

Đặt \(\left(\frac{1}{x};\frac{1}{y};\frac{1}{z}\right)=\left(a;b;c\right)\left(a,b,c>0\right)\)

Khi đó: \(H=\frac{a}{9b^2+1}+\frac{b}{9c^2+1}+\frac{c}{9a^2+1}\)

\(=\left(a+b+c\right)-\left(\frac{9ab^2}{9b^2+1}+\frac{9bc^2}{9c^2+1}+\frac{9ca^2}{9a^2+1}\right)\)

\(\ge1-\left(\frac{9ab^2}{6b}+\frac{9bc^2}{6c}+\frac{9ca^2}{6a}\right)\)

\(=1-\frac{3}{2}\left(ab+bc+ca\right)\ge1-\frac{3}{2}\cdot\frac{\left(a+b+c\right)^2}{3}=1-\frac{3}{2}\cdot\frac{1}{3}=\frac{1}{2}\)

Dấu "=" xảy ra khi: \(x=y=z=3\)

Vậy Min(H) = 1/2 khi x = y = z = 3

\(\sqrt{32-10\sqrt{7}}-\sqrt{43-12\sqrt{7}}\)

\(=\sqrt{32-2.5\sqrt{7}}-\sqrt{43-2.6\sqrt{7}}\)

\(=\sqrt{25-2.5\sqrt{7}+7}-\sqrt{36-2.6\sqrt{7}+7}\)

\(=\sqrt{\left(5-\sqrt{7}\right)^2}-\sqrt{\left(6-\sqrt{7}\right)^2}\)

\(=5-\sqrt{7}-6+\sqrt{7}=-1\)