Let x, y, z be positive real numbers such that xy + yz + zx + xyz = 4 . Prove that :
\(3\left(\frac{1}{\sqrt{x}}+\frac{1}{\sqrt{y}}+\frac{1}{\sqrt{z}}\right)^2\ge\left(x+2\right)\left(y+2\right)\left(z+2\right)\)
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Không mất tính tổng quát giả sử \(c=max\left\{a,b,c\right\}\)
\(\Rightarrow2c\ge a+b\)
\(\Rightarrow c\ge\frac{a+b}{2}\)
Từ giả thiết \(\Rightarrow a,b\le1\)
\(\Rightarrow ab\le1\)( *)
Đặt \(P=\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}-\frac{5}{2}\)
\(=\frac{1}{a+b}+\frac{1}{b+\frac{1-ab}{a+b}}+\frac{1}{a+\frac{1-ab}{a+b}}-\frac{5}{2}\)
Đặt \(S=\frac{1}{a+b+\frac{1}{a+b}}+a+b+\frac{1}{a+b}-\frac{5}{2}\)
Xét hiệu \(P-S=\)\(\frac{1}{a+b}+\frac{1}{b+\frac{1-ab}{a+b}}+\frac{1}{a+\frac{1-ab}{a+b}}-\frac{5}{2}-\)\(-\frac{1}{a+b+\frac{1}{a+b}}-a-b-\frac{1}{a+b}+\frac{5}{2}\)
\(=\frac{1}{\frac{ab+b^2+1-ab}{a+b}}+\frac{1}{\frac{a^2+ab+1-ab}{a+b}}-\frac{1}{\frac{\left(a+\right)^2+1}{a+b}}-\left(a+b\right)\)
\(=\frac{a+b}{b^2+1}+\frac{a+b}{c^2+1}-\left(a+b\right)\left[1+\frac{1}{1+\left(a+b\right)^2}\right]\)
Ta sẽ chứng minh \(\frac{a+b}{b^2+1}+\frac{a+b}{c^2+1}-\left(a+b\right)\left[1+\frac{1}{1+\left(a+b\right)^2}\right]\ge0\)
\(\Leftrightarrow\frac{a+b}{b^2+1}+\frac{a+b}{c^2+1}\ge\left(a+b\right)\left[1+\frac{1}{1+\left(a+b\right)^2}\right]\)
\(\Leftrightarrow\frac{1}{b^2+1}+\frac{1}{c^2+1}\ge1+\frac{1}{1+\left(a+b\right)^2}\)
\(\Leftrightarrow\frac{2+a^2+b^2}{\left(1+a^2\right)\left(1+b^2\right)}\ge\frac{2+\left(a+b\right)^2}{1+\left(a+b\right)^2}\)
\(\Rightarrow\left(2+b^2+a^2\right)\left[1+\left(a+b\right)^2\right]\ge\left[2+\left(a+b\right)^2\right]\left(1+a^2\right)\left(1+b^2\right)\)
\(\Leftrightarrow2+2\left(a+b\right)^2+\left(a+b\right)^2\left(a^2+b^2\right)+a^2+b^2\ge\left[2+\left(a+b\right)^2\right]\left(1+a^2+b^2+a^2b^2\right)\)
\(\Leftrightarrow2+2\left(a+b\right)^2+\left(a+b\right)^2\left(a^2+b^2\right)+a^2+b^2-2a^2b^2-\left(a+b\right)^2\left(a^2+b^2\right)-\left(a+b\right)^2a^2b^2\)\(-2-2\left(a^2+b^2\right)-\left(a+b^2\right)\ge0\)
\(\Leftrightarrow-2a^2b^2-\left(a+b\right)^2a^2b^2+a^2+b^2-\left(a+b\right)^2\ge0\)
\(\Leftrightarrow ab\left[ab\left(a+b\right)^2+2ab-2\right]\le0\)
\(\Leftrightarrow ab\left(a+b\right)^2+2ab-2\le0\)( do a,b \(\ge0\))
\(\Leftrightarrow ab\left(a+b\right)^2\le2\left(1-ab\right)\)
\(\Leftrightarrow ab\left(a+b\right)^2\le2c\left(a+b\right)\) (1)
Mà \(c\ge\frac{a+b}{2}\)
\(\Rightarrow2c\left(a+b\right)\ge\left(a+b\right)^2\)
Ta có: \(\left(a+b\right)^2\ge ab\left(a+b\right)^2\)
\(\Leftrightarrow\left(a+b\right)^2\left(1-ab\right)\ge0\)( đúng do (*) )
\(\Rightarrow\left(1\right)\)đúng
\(\Rightarrow P-S\ge0\)
\(\Rightarrow P\ge S\)
Ta phải chứng minh \(S\ge0\)
\(\Leftrightarrow\frac{1}{a+b+\frac{1}{a+b}}+a+b+\frac{1}{a+b}\ge\frac{5}{2}\)
\(\Leftrightarrow\frac{a+b}{1+\left(a+b\right)^2}+\frac{1+\left(a+b\right)^2}{a+b}\ge\frac{5}{2}\) (2)
Đặt \(x=\frac{1+\left(a+b\right)^2}{a+b}\)
Ta có: \(1+\left(a+b\right)^2\ge2\left(a+b\right)\)
\(\Leftrightarrow\left(a+b-1\right)^2\ge0\)( đúng )
\(\Rightarrow x=\frac{1+\left(a+b\right)^2}{a+b}\ge2\)
=> (2) có dạng \(x+\frac{1}{x}\ge\frac{5}{2}\)
\(\Leftrightarrow2x^2-5x+2\ge0\)
\(\Leftrightarrow\left(2x-1\right)\left(x-2\right)\ge0\)( đúng )
\(\Rightarrow S\ge0\)mà \(P\ge S\)
\(\Rightarrow P\ge0\)
\(\Leftrightarrow\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\ge\frac{5}{2}\)
Dấu "=" xảy ra \(\Leftrightarrow\hept{\begin{cases}a+b=1\\ab+bc+ca=1\\ab\left[ab\left(a+b\right)^2+2ab-2\right]=0\end{cases}}\)
\(\Leftrightarrow\orbr{\begin{cases}a=c=1;b=0\\b=c=1;a=0\end{cases}}\)
a, Ta có \(\sqrt{25-16}=\sqrt{9}=3\)
\(\sqrt{25}-\sqrt{16}=5-4=1\)
Do 3 > 1 nên \(\sqrt{25-16}>\sqrt{25}-\sqrt{16}\)
a) căn 25 - 16 > căn 25 - căn 16
b)Với nên đều xác định
Để so sánh và ta quy về so sánh và .
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Do nên
Do
(đpcm)
Vậy .
(Vì x > 0 nên |x| = x; y2 > 0 với mọi y ≠ 0)
(Vì x2 ≥ 0 với mọi x; và vì y < 0 nên |2y| = – 2y)
(Vì x < 0 nên |5x| = – 5x; y > 0 nên |y3| = y3)
(Vì x2y4 = (xy2)2 > 0 với mọi x ≠ 0, y ≠ 0)
a, \(\frac{\sqrt{2}}{\sqrt{18}}=\sqrt{\frac{2}{18}}=\sqrt{\frac{1}{9}}=\frac{1}{3}\)
b, \(\frac{\sqrt{15}}{\sqrt{735}}=\sqrt{\frac{15}{735}}=\sqrt{\frac{1}{49}}=\frac{1}{7}\)
c, \(\frac{\sqrt{12500}}{\sqrt{500}}=\sqrt{\frac{12500}{500}}=\sqrt{\frac{125}{5}}=\sqrt{25}=5\)
d, \(\frac{\sqrt{6^5}}{\sqrt{2^3.3^5}}=\sqrt{\frac{6^5}{2^3.3^5}}=\sqrt{\frac{2^5.3^5}{2^3.3^5}}=\sqrt{2^2}=2\)
a) căn 2 / căn 18 = 1/3
b) căn 15/ căn 735 = 1/7
c) căn 12500 / căn 500 = 5
d) căn 6^5 / 2^3 * 3^5 = 2
a, \(\sqrt{\frac{289}{25}}=\frac{\sqrt{289}}{\sqrt{25}}=\frac{17}{5}\)
b, \(\sqrt{2\frac{14}{25}}=\sqrt{\frac{64}{25}}=\frac{8}{5}\)
c, \(\sqrt{\frac{0,25}{9}}=\frac{\sqrt{0,25}}{\sqrt{9}}=\frac{0,5}{3}=\frac{1}{2}.\frac{1}{3}=\frac{1}{6}\)
d, \(\sqrt{\frac{8,1}{16}}\)đề có sai ko cô ?
a) căn 289 / 225 = 17/15
b) căn 64/ 25 = 8/5
c) căn 0,25 / 9 = 1/6
d) căn 8,1 / 1,6 = 9/4
Đặt \(A=\frac{a^3}{a^2+b^2}+\frac{b^3}{b^2+c^2}+\frac{c^3}{c^2+a^2}\left(a,b,c>0\right)\).
Ta có:
\(\frac{a^3}{a^2+b^2}=\frac{a\left(a^2+b^2-b^2\right)}{a^2+b^2}=\frac{a\left(a^2+b^2\right)-ab^2}{a^2+b^2}=a-\frac{ab^2}{a^2+b^2}\).
Vì \(a,b>0\)nên áp dụng bất đẳng thức Cô-si cho 2 số dương, ta được:
\(a^2+b^2\ge2ab\).
\(\Rightarrow\frac{1}{a^2+b^2}\le\frac{1}{2ab}\).
\(\Leftrightarrow\frac{ab^2}{a^2+b^2}\le\frac{ab^2}{2ab}=\frac{b}{2}\).
\(\Rightarrow\frac{-ab^2}{a^2+b^2}\ge\frac{-b}{2}\).
\(\Leftrightarrow a-\frac{ab^2}{a^2+b^2}\ge a-\frac{b}{2}\).
\(\Leftrightarrow\frac{a^3}{a^2+b^2}\ge a-\frac{b}{2}\left(1\right)\).
Dấu bằng xảy ra \(\Leftrightarrow a=b>0\).
Chứng minh tương tự, ta được:
\(\frac{b^3}{b^2+c^2}\ge b-\frac{c}{2}\).với \(b,c>0\)\(\left(2\right)\)
Dấu bẳng xảy ra \(\Leftrightarrow b=c>0\).
Chứng minh tương tự, ta được:
\(\frac{c^3}{c^2+a^2}\ge c-\frac{a}{2}\)với \(a,c>0\)\(\left(3\right)\).
Dấu bằng xảy ra \(\Leftrightarrow a=c>0\).
Từ \(\left(1\right),\left(2\right),\left(3\right)\), ta được:
\(\frac{a^3}{a^2+b^2}+\frac{b^3}{b^2+c^2}+\frac{c^3}{c^2+a^2}\)\(\ge\)\(a+b+c-\frac{a}{2}-\frac{b}{2}-\frac{c}{2}\).
\(\Leftrightarrow A\ge\frac{a+b+c}{2}\).
\(\Leftrightarrow A\ge\frac{6}{2}\)(vì \(a+b+c=6\)).
\(\Leftrightarrow A\ge3\)(điều phải chứng minh).
Dấu bằng xảy ra.
\(\Leftrightarrow\hept{\begin{cases}a=b=c>0\\a+b+c=6\end{cases}}\Leftrightarrow a=b=c=2\).
Vậy nếu \(a,b,c\)là các số thực dương thỏa mãn \(a+b+c=6\)thì:
\(\frac{a^3}{a^2+b^2}+\frac{b^3}{b^2+c^2}+\frac{c^3}{c^2+a^2}\ge3\).
Đặt \(x=\frac{2a}{b+c};y=\frac{2b}{c+a};z=\frac{2c}{a+b}\) Thì bài toán thành chứng minh
\(3\left(\sqrt{\frac{a+b}{2c}}+\sqrt{\frac{b+c}{2a}}+\sqrt{\frac{c+a}{2b}}\right)^2\ge\frac{8\left(a+b+c\right)^3}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}\)
Áp dụng holder ta có:
\(\left(\sqrt{\frac{a+b}{2c}}+\sqrt{\frac{b+c}{2a}}+\sqrt{\frac{c+a}{2b}}\right)^2\left(2c\left(a+b\right)^2+2a\left(b+c\right)^2+2b\left(c+a\right)^2\right)\)
\(\ge\left[\left(a+b\right)+\left(b+c\right)+\left(c+a\right)\right]^3=8\left(a+b+c\right)^3\)
\(\Rightarrow VT\ge3.\frac{8\left(a+b+c\right)^3}{2a\left(b+c\right)^2+2b\left(c+a\right)^2+2c\left(a+b\right)^2}\)
Từ đây ta cần chứng minh:
\(3.\frac{8\left(a+b+c\right)^3}{2a\left(b+c\right)^2+2b\left(c+a\right)^2+2c\left(a+b\right)^2}\ge\frac{8\left(a+b+c\right)^3}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}\)
\(\Leftrightarrow2a\left(b+c\right)^2+2b\left(c+a\right)^2+2c\left(a+b\right)^2\le3\left(a+b\right)\left(b+c\right)\left(c+a\right)\)
\(\Leftrightarrow a\left(b-c\right)^2+b\left(c-a\right)^2+c\left(a-b\right)^2\ge0\)( đúng )
Vậy có ĐPCM