Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
Bài này thiếu đề. Đề đúng là phải có \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=1\) nữa nha bạn.
\(\frac{a^2}{a+bc}+\frac{b^2}{b+ac}+\frac{c^2}{c+ab}\ge\frac{a+b+c}{4}\)
Ta có: \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=1\) \(\Rightarrow ab+bc+ac=abc\)
\(VT=\frac{a^2}{a+bc}+\frac{b^2}{b+ac}+\frac{c^2}{c+ab}\)
\(\Rightarrow VT=\frac{a^2.a}{a\left(a+bc\right)}+\frac{b^2.b}{b\left(b+ac\right)}+\frac{c^2.c}{c\left(c+ab\right)}\)
\(\Leftrightarrow VT=\frac{a^3}{a^2+abc}+\frac{b^3}{b^2+abc}+\frac{c^3}{c^2+abc}\)
\(\Leftrightarrow VT=\frac{a^3}{a^2+ab+bc+ac}+\frac{b^3}{b^2+ab+bc+ac}+\frac{c^3}{c^2+ab+bc+ac}\)
\(\Leftrightarrow VT=\frac{a^3}{a\left(a+b\right)+c\left(a+b\right)}+\frac{b^3}{a\left(b+c\right)+b\left(b+c\right)}+\frac{c^3}{c\left(b+c\right)+a\left(b+c\right)}\)
\(\Leftrightarrow VT=\frac{a^3}{\left(a+c\right)\left(a+b\right)}+\frac{b^3}{\left(b+c\right)\left(a+b\right)}+\frac{c^3}{\left(b+c\right)\left(a+c\right)}\)
Áp dụng BĐT Cauchy ta có:
\(\frac{a^3}{\left(a+b\right)\left(a+c\right)}+\frac{a+b}{8}+\frac{a+c}{8}\ge3\sqrt[3]{\frac{a^3}{64}}=\frac{3a}{4}\)
\(\frac{b^3}{\left(a+b\right)\left(b+c\right)}+\frac{a+b}{8}+\frac{b+c}{8}\ge3\sqrt[3]{\frac{b^3}{64}}=\frac{3b}{4}\)
\(\frac{c^3}{\left(b+c\right)\left(a+c\right)}+\frac{b+c}{8}+\frac{a+c}{8}\ge3\sqrt[3]{\frac{c^3}{64}}=\frac{3c}{4}\)
Ta có:
\(\frac{3a}{4}+\frac{3b}{4}+\frac{3c}{4}+\frac{a+b+c}{2}\ge\frac{3}{4}\left(a+b+c\right)\)
\(\Rightarrow\frac{3a}{4}+\frac{3b}{4}+\frac{3c}{4}\ge\frac{3}{4}\left(a+b+c\right)-\frac{1}{2}\left(a+b+c\right)\)
\(\Rightarrow VT\ge\frac{a+b+c}{4}=VP\)
Dấu \("="\) xảy ra \(\Leftrightarrow a=b=c=3\)
\(\RightarrowĐpcm.\)
\(x^4+\sqrt{x^2+3}=3\)
\(\Leftrightarrow x^4-1+\sqrt{x^2+3}-2=0\)
\(\Leftrightarrow\left(x-1\right)\left(x+1\right)\left(x^2+1\right)+\frac{x^2+3-4}{\sqrt{x^2+3}+2}=0\)
\(\Leftrightarrow\left(x-1\right)\left(x+1\right)\left(x^2+1\right)+\frac{\left(x+1\right)\left(x-1\right)}{\sqrt{x^2+3}+2}=0\)
\(\Leftrightarrow\left(x-1\right)\left(x+1\right)\left(x^2+1+\frac{1}{\sqrt{x^2+3}+2}\right)=0\)
\(\Leftrightarrow\left(x-1\right)\left(x+1\right)=0\)vì \(x^2+1+\frac{1}{\sqrt{x^2+3}+2}>0\)
\(\Leftrightarrow\int^{x=1}_{x=-1}\)
\(a+b+c+ab+ac+bc=6abc\Rightarrow\frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{ab}+\frac{1}{ac}+\frac{1}{bc}=6\)
Đặt \(\frac{1}{a}=x;\frac{1}{b}=y;\frac{1}{c}=z\left(x;y;z>0\right)\)
Ta được: \(x+y+z+xy+xz+yz=6\)
Ta đi chứng minh: \(x^2+y^2+z^2\ge3\)
Có: \(x^2+1\ge2x;y^2+1\ge2y;z^2+1\ge2z\)(Cô-si)
\(\Rightarrow x^2+y^2+z^2+3\ge2\left(x+y+z\right)\)(1)
Dấu "=" xảy ra <=> x=y=z=1
\(x^2+y^2\ge2xy;y^2+z^2\ge2yz;x^2+z^2\ge2xz\)(Cô-si)
\(\Rightarrow2x^2+2y^2+2z^2\ge2\left(xy+xz+yz\right)\)(2)
Dấu "=" xảy ra <=> x=y=z
cộng vế với vế của (1) và (2)
\(\Rightarrow3\left(x^2+y^2+z^2\right)+3\ge2\left(x+y+z+xy+xz+yz\right)=12\)
\(\Rightarrow x^2+y^2+z^2\ge3\)
Dấu "=" xảy ra <=> x=y=z=1<=>a=b=c=1
Nhớ tick nhé