- tim cac so nguyen a;b;c sao cho: a^2+b^2+c^2+4<hoac= ab+3b+2c
2. giai phuong trinh: \(\sqrt{2x+3}+\sqrt{5-2x}=3x^2-12x+14\)(neu cach giai)
3. tim gia tri nho nhat cua: \(\frac{x+8}{\sqrt{x}+1}\)
4. tim gia tri nho nhat cua: \(\frac{4a}{b+c-a}+\frac{9b}{a+c-b}+\frac{16c}{a+b-c}\)
5. cho a;b;c la 3 canh cua tam giac thoa man a+b+c=2 ; 0<a;b;c<1 c/m a^2+b^2+c^2+2abc<2
6. giai he phuong trinh 6(x+y)=5xy ; 12(y+z)=7zy ; 4(z+x)=3xz
7. cho a; b;c la 3 canh cua 1 tam giac c/m voi moi x,y,z \(\frac{x^2}{a^2}+\frac{y^2}{b^2}+\frac{z^2}{c^2}>\frac{2\left(x^2+y^2+z^2\right)}{a^2+b^2+c^2}\)
8. cho x;y;z>0 thoa man x+y+z=2008 c/m \(\frac{x^4+y^4}{x^3+y^3}+\frac{y^4+z^4}{y^3+z^3}+\frac{z^4+x^4}{z^3+x^3}>hoac=2008\)
2)đk: x>=0 \(\frac{x+8}{\sqrt{x}+1}=\frac{x-1+9}{\sqrt{x}+1}=\frac{\left(\sqrt{x}-1\left(\sqrt{x}+1\right)\right)}{\sqrt{x}+1}+\frac{9}{\sqrt{x}+1}=\sqrt{x}-1+\frac{9}{\sqrt{x}+1}=\sqrt{x}+1+\frac{9}{\sqrt{x}+1}-2\)
\(x\ge0\Leftrightarrow\sqrt{x}\ge0\Rightarrow\sqrt{x}+1>0;\frac{9}{\sqrt{x}+1}>0\). áp dụng bđt cosi cho 2 số dương \(\sqrt{x}+1;\frac{9}{\sqrt{x}+1}\) ta có:
\(\sqrt{x}+1+\frac{9}{\sqrt{x}+1}\ge2\sqrt{9}=6\Leftrightarrow\sqrt{x}+1+\frac{9}{\sqrt{x}+1}-2\ge6-2=4\)=> Min =4 <=> x=4.
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