Áp dụng BĐT: x2+y2+z2\(\ge\)xy+yz+zx ( với x,y,z >0)
Ta có\(\dfrac{a^8+b^8+c^8}{a^3b^3c^3}\)\(\ge\)\(\dfrac{a^4b^4+b^4c^4+c^4a^4}{a^3b^3c^3}\)
\(\ge\)\(\dfrac{a^4b^2c^2+b^4c^2a^2+c^4a^2b^2}{a^3b^3c^3}\)=\(\dfrac{a^2+b^2+c^2}{abc}\)\(\ge\)\(\dfrac{ab+bc+ca}{abc}\)
= \(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\) (đpcm)
Dấu "=" xảy ra \(\Leftrightarrow\) a=b=c
Bài 1: Cho x, y, z > 0 thỏa mãn xyz = 1. Chứng minh rằng: \(\dfrac{1}{x^3\left(y+z\right)}+\dfrac{1}{y^3\left(z+x\right)}+\dfrac{1}{z^3\left(x+y\right)}>=\dfrac{3}{2}\)
Bài 2: Cho a, b c > 0. Chứng minh rằng: \(\dfrac{a+3c}{a+b}+\dfrac{c+3a}{b+c}+\dfrac{4b}{c+a}>=6\)
Bài 1 : cho x, y >0 và x2+y2=1. Tìm GTNN của \(P=\left(1+x\right)\cdot\left(1+\dfrac{1}{y}\right)+\left(1+y\right)\cdot\left(1+\dfrac{1}{x}\right)\)
Bài 2 : cho a, b, c > 0. CMR
\(\dfrac{1}{a+3b}+\dfrac{1}{b+3c}+\dfrac{1}{c+3a}>=\dfrac{1}{2a+b+c}+\dfrac{1}{2b+a+c}+\dfrac{1}{2c+a+b}\)
Bài 3 : cho a, b, c, d >0. CMR
\(\dfrac{a+c}{a+b}+\dfrac{b+d}{b+c}+\dfrac{c+a}{c+d}+\dfrac{d+b}{d+a}>=4\)
cho a,b,c>0 chứng minh rằng
\(\dfrac{1}{\left(a-b\right)^2}+\dfrac{1}{\left(b-c\right)^2}+\dfrac{1}{\left(c-a\right)^2}\ge\dfrac{4}{ab+bc+ca}\)
Cho a,b,c >0. Chứng minh \(\dfrac{1}{\left(2a+b\right)\left(2a+c\right)}+\dfrac{1}{\left(2b+c\right)\left(2b+a\right)}+\dfrac{1}{\left(2c+a\right)\left(2c+b\right)}\ge\dfrac{1}{ab+bc+ca}\)
Bài 1 : Cho a, b, c > 0. Chứng minh rằng : \(\dfrac{a^2}{b^3}+\dfrac{b^2}{c^3}+\dfrac{c^2}{a^3}>=\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\)
Bài 2 : Cho a, b, c là các số thực. Chứng minh rằng : a4+b4+c4 >= abc(a+b+c)
cho các số dương a,b,c thỏa mãn abc=1. chứng minh rằng
\(\dfrac{a^3}{\left(1+b\right)\left(1+c\right)}+\dfrac{b^3}{\left(1+c\right)\left(1+a\right)}+\dfrac{c^3}{\left(1+a\right)\left(1+c\right)}\ge\dfrac{3}{4}\)
cho a,b,c >0 chứng minh rằng \(\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a}>=\dfrac{a+b}{b+c}+\dfrac{b+c}{a+b}+1\)
Cho các biểu thức:
A= \(\dfrac{1}{\sqrt{1}+\sqrt{2}}+\dfrac{1}{\sqrt{2}+\sqrt{3}}+\dfrac{1}{\sqrt{3}+\sqrt{4}}+...+\dfrac{1}{\sqrt{24}+\sqrt{25}}\)
B= \(\dfrac{1}{\sqrt{1}}+\dfrac{1}{\sqrt{2}}+\dfrac{1}{\sqrt{3}}+...+\dfrac{1}{\sqrt{24}}\)
a. Trục căn ở mẫu biểu thức \(\dfrac{1}{\sqrt{n+1}+\sqrt{n}}\)
b. Tính giá trị của A
c. Chứng minh rằng B>8
từ giả thiết, ta có \(\dfrac{1}{xy}+\dfrac{1}{yz}+\dfrac{1}{zx}=1\)
đặt \(\left(\dfrac{1}{xy};\dfrac{1}{yz};\dfrac{1}{zx}\right)=\left(a;b;c\right)\Rightarrow a+b+c=1\) =>\(\left(\dfrac{ac}{b};\dfrac{ab}{c};\dfrac{bc}{a}\right)=\left(\dfrac{1}{x^2};\dfrac{1}{y^2};\dfrac{1}{z^2}\right)\)
ta có VT=\(\dfrac{1}{\sqrt{1+\dfrac{1}{x^2}}}+\dfrac{1}{\sqrt{1+\dfrac{1}{y^2}}}+\dfrac{1}{\sqrt{1+\dfrac{1}{z^1}}}=\sqrt{\dfrac{1}{1+\dfrac{ac}{b}}}+\sqrt{\dfrac{1}{1+\dfrac{ab}{c}}}+\sqrt{\dfrac{1}{1+\dfrac{bc}{a}}}\)
=\(\dfrac{1}{\sqrt{\dfrac{b+ac}{b}}}+\dfrac{1}{\sqrt{\dfrac{a+bc}{a}}}+\dfrac{1}{\sqrt{\dfrac{c+ab}{c}}}=\sqrt{\dfrac{a}{\left(a+b\right)\left(a+c\right)}}+\sqrt{\dfrac{b}{\left(b+c\right)\left(b+a\right)}}+\sqrt{\dfrac{c}{\left(c+a\right)\left(c+b\right)}}\)
\(\le\sqrt{3}\sqrt{\dfrac{ac+ab+bc+ba+ca+cb}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}}=\sqrt{3}.\sqrt{\dfrac{2\left(ab+bc+ca\right)}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}}\)
ta cần chứng minh \(\sqrt{\dfrac{2\left(ab+bc+ca\right)}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}}\le\dfrac{3}{2}\Leftrightarrow\dfrac{2\left(ab+bc+ca\right)}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}\le\dfrac{9}{4}\Leftrightarrow8\left(ab+bc+ca\right)\le9\left(a+b\right)\left(b+c\right)\left(c+a\right)\)
<=>\(8\left(a+b+c\right)\left(ab+bc+ca\right)\le9\left(a+b\right)\left(b+c\right)\left(c+a\right)\) (luôn đúng )
^_^
