cho\(\left\{{}\begin{matrix}a\ge2\\b\ge3\\c\ge6\end{matrix}\right.\)
tìm max p= \(\dfrac{bc\sqrt{a-2}+ca\sqrt[3]{b-3}+ab\sqrt[4]{c-6}}{abc}\)
Cho a, b, c thỏa mãn \(\left\{{}\begin{matrix}a\ge3\\b\ge4\\c\ge2\end{matrix}\right.\)
Tìm Max \(y=\dfrac{ab\sqrt{c-2}+bc\sqrt{a-3}+ac\sqrt{b-4}}{abc}\)
\(\Leftrightarrow y=\dfrac{\sqrt{c-2}}{c}+\dfrac{\sqrt{a-3}}{a}+\dfrac{\sqrt{b-4}}{b}\)
Ta có: \(\dfrac{\sqrt{c-2}}{c}\le\dfrac{1}{2\sqrt{2}}\Leftrightarrow\left(\sqrt{c-2}-\sqrt{2}\right)^2\ge0\) ( Luôn đúng)
Tương tự: \(\dfrac{\sqrt{a-3}}{a}\le\dfrac{1}{2\sqrt{3}};\dfrac{\sqrt{b-4}}{b}\le\dfrac{1}{4}\)
\(\Rightarrow y\le\dfrac{1}{2\sqrt{2}}+\dfrac{1}{2\sqrt{3}}+\dfrac{1}{4}\) và dấu ''='' xảy ra khi c = 4; a = 6; b = 8
1. Cho \(\left\{{}\begin{matrix}a,b,c\ge0\\a+b+c=1_{ }\end{matrix}\right.\). Chứng minh rằng: \(\sqrt{a+b}+\sqrt{b+c}+\sqrt{c+a}\le\sqrt{6}\)
2. Cho \(\left\{{}\begin{matrix}a\ge3\\b\ge4\\c\ge2\end{matrix}\right.\). Tìm giá trị lớn nhất của biểu thức P=\(\dfrac{ab\sqrt{c-2}+bc\sqrt{a-3}+ca\sqrt{b-4}}{2\sqrt{2}}\)
3. Cho \(x,y>0\). Tìm giá trị nhỏ nhất của biểu thức: \(f\left(x;y\right)=\dfrac{\left(x+y\right)^3}{xy^2}\)
1. a) \(\left\{{}\begin{matrix}x,y,z>0\\xyz=1\end{matrix}\right.\). Tìm max \(P=\frac{1}{\sqrt{x^5-x^2+3xy+6}}+\frac{1}{\sqrt{y^5-y^2+3yz+6}}+\frac{1}{\sqrt{z^5-z^2+zx+6}}\)
b) \(\left\{{}\begin{matrix}x,y,z>0\\xyz=8\end{matrix}\right.\). Min \(P=\frac{x^2}{\sqrt{\left(1+x^3\right)\left(1+y^3\right)}}+\frac{y^2}{\sqrt{\left(1+y^3\right)\left(1+z^3\right)}}+\frac{z^2}{\sqrt{\left(1+z^3\right)\left(1+x^3\right)}}\)
c) \(x,y,z>0.\) Min \(P=\sqrt{\frac{x^3}{x^3+\left(y+z\right)^3}}+\sqrt{\frac{y^3}{y^3+\left(z+x\right)^3}}+\sqrt{\frac{z^3}{z^3+\left(x+y\right)^3}}\)
d) \(a,b,c>0;a^2+b^2+c^2+abc=4.Cmr:2a+b+c\le\frac{9}{2}\)
e) \(\left\{{}\begin{matrix}a,b,c>0\\a+b+c=3\end{matrix}\right.\). Cmr: \(\frac{a}{b^3+ab}+\frac{b}{c^3+bc}+\frac{c}{a^3+ca}\ge\frac{3}{2}\)
f) \(\left\{{}\begin{matrix}a,b,c>0\\ab+bc+ca+abc=4\end{matrix}\right.\) Cmr: \(\sqrt{ab}+\sqrt{bc}+\sqrt{ca}\le3\)
g) \(\left\{{}\begin{matrix}a,b,c>0\\ab+bc+ca+abc=2\end{matrix}\right.\) Max : \(Q=\frac{a+1}{a^2+2a+2}+\frac{b+1}{b^2+2b+2}+\frac{c+1}{c^2+2c+2}\)
Câu c quen thuộc, chém trước:
Ta có BĐT phụ: \(\frac{x^3}{x^3+\left(y+z\right)^3}\ge\frac{x^4}{\left(x^2+y^2+z^2\right)^2}\) \((\ast)\)
Hay là: \(\frac{1}{x^3+\left(y+z\right)^3}\ge\frac{x}{\left(x^2+y^2+z^2\right)^2}\)
Có: \(8(y^2+z^2) \Big[(x^2 +y^2 +z^2)^2 -x\left\{x^3 +(y+z)^3 \right\}\Big]\)
\(= \left( 4\,x{y}^{2}+4\,x{z}^{2}-{y}^{3}-3\,{y}^{2}z-3\,y{z}^{2}-{z}^{3 } \right) ^{2}+ \left( 7\,{y}^{4}+8\,{y}^{3}z+18\,{y}^{2}{z}^{2}+8\,{z }^{3}y+7\,{z}^{4} \right) \left( y-z \right) ^{2} \)
Từ đó BĐT \((\ast)\) là đúng. Do đó: \(\sqrt{\frac{x^3}{x^3+\left(y+z\right)^3}}\ge\frac{x^2}{x^2+y^2+z^2}\)
\(\therefore VT=\sum\sqrt{\frac{x^3}{x^3+\left(y+z\right)^3}}\ge\sum\frac{x^2}{x^2+y^2+z^2}=1\)
Done.
Câu 1 chuyên phan bội châu
câu c hà nội
câu g khoa học tự nhiên
câu b am-gm dựa vào hằng đẳng thử rồi đặt ẩn phụ
câu f đặt \(a=\frac{2m}{n+p};b=\frac{2n}{p+m};c=\frac{2p}{m+n}\)
Gà như mình mấy câu còn lại ko bt nha ! để bạn tth_pro full cho nhé !
Nguyễn Ngọc Lộc , ?Amanda?, Phạm Lan Hương, Akai Haruma, @Trần Thanh Phương, @Nguyễn Việt Lâm,
Giúp em vs ạ! Thanks nhiều ạ
tìm max s= \(\dfrac{\sqrt[3]{\left(a-2\right)\left(b-3\right)}}{a+b}\) với \(\left\{{}\begin{matrix}a\ge2\\b\ge3\end{matrix}\right.\)
\(S=\dfrac{\sqrt[3]{\left(a-2\right)\left(b-3\right)}}{a+b}\)
\(\Rightarrow S.\sqrt[3]{5}=\dfrac{\sqrt[3]{\left(a-2\right)\left(b-3\right).5}}{a+b}\)
\(\le\dfrac{\dfrac{\left(a-2\right)+\left(b-3\right)+5}{3}}{a+b}=\dfrac{\dfrac{a+b}{3}}{a+b}=\dfrac{1}{3}\)
\(\Rightarrow S\le\dfrac{1}{3}:\sqrt[3]{5}=\dfrac{1}{3\sqrt[3]{5}}\)
Đẳng thức xảy ra \(\Leftrightarrow a-2=b-3=5\Leftrightarrow\left\{{}\begin{matrix}a=7\\b=8\end{matrix}\right.\)
cho \(\left\{{}\begin{matrix}a,b,c>0\\ab+bc+ca\ge3\end{matrix}\right.\)
cmr \(\sqrt{a+3}+\sqrt{b+3}+\sqrt{c+3}\le2\left(a^2+b^2+c^2\right)\)
Ta có BĐT \(3\left(a^2+b^2+c^2\right)\ge\left(a+b+c\right)^2\)
\(\Rightarrow a+b+c\le\sqrt{3\left(a^2+b^2+c^2\right)}\)
Lợi dụng BĐT Cauchy-Schwarz tao cso:
\(VT^2=\left(\sqrt{a+3}+\sqrt{b+3}+\sqrt{c+3}\right)^2\)
\(\le\left(1+1+1\right)\left(a+b+c+9\right)\)
\(\le3\left(\sqrt{3\left(a^2+b^2+c^2\right)}+9\right)\)
Đặt \(t=a^2+b^2+c^2\left(t\ge3\right)\) thì cần chứng minh:
\(3\left(\sqrt{3\left(a^2+b^2+c^2\right)}+9\right)\le4\left(a^2+b^2+c^2\right)^2\)
\(\Leftrightarrow3\left(a^2+b^2+c^2+9\right)\le4\left(a^2+b^2+c^2\right)^2\)
\(\Leftrightarrow3\left(t+9\right)\le4t^2\Leftrightarrow-\left(t-3\right)\left(4t+9\right)\le0\) (Đúng)
Ta có BĐT \(3\le ab+bc+ca\le a^2+b^2+c^2\)
Và BĐT: \(3\left(a^2+b^2+c^2\right)\ge\left(a+b+c\right)^2\)
\(\Rightarrow a+b+c\le\sqrt{3\left(a^2+b^2+c^2\right)}\)
\(\le\sqrt{9}=3\le a^2+b^2+c^2\)
Áp dụng BĐT Cauchy-Schwarz ta có:
\(VT^2=\left(\sqrt{a+3}+\sqrt{b+3}+\sqrt{c+3}\right)^2\)
\(\le\left(1+1+1\right)\left(a+b+c+9\right)\)
\(\le\left(a^2+b^2+c^2\right)\left[a^2+b^2+c^2+3\left(a^2+b^2+c^2\right)\right]\)
\(=4\left(a^2+b^2+c^2\right)=VP^2\)
Xảy ra khi \(a=b=c=1\)
cho a,b,c thỏa \(\left\{{}\begin{matrix}a,b,c>0\\\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=1\end{matrix}\right.\) chứng minh rằng\(\sqrt{a+bc}+\sqrt{b+ca}+\sqrt{c+ab}\ge\sqrt{abc}+\sqrt{a}+\sqrt{b}+\sqrt{\sqrt{c}}\)
\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=1\Leftrightarrow ab+bc+ca=abc\)
Ta có: \(\sqrt{a+bc}=\sqrt{\dfrac{a^2+abc}{a}}=\sqrt{\dfrac{\left(a+b\right)\left(a+c\right)}{a}}\)
thiết lập tương tự ,bất đẳng thức cần chứng minh tương đương:
\(\Leftrightarrow\sum\sqrt{\dfrac{\left(a+b\right)\left(a+c\right)}{a}}\ge\sqrt{abc}+\sqrt{a}+\sqrt{b}+\sqrt{c}\)
\(\Leftrightarrow\sum\sqrt{bc\left(a+b\right)\left(a+c\right)}\ge abc+\sqrt{abc}\left(\sqrt{a}+\sqrt{b}+\sqrt{c}\right)\)
\(\Leftrightarrow\sum\sqrt{\left(b^2+ab\right)\left(c^2+ac\right)}\ge abc+\sum a\sqrt{bc}\)
Điều này luôn đúng theo BĐT Bunyakovsky:
\(\sum\sqrt{\left(b^2+ab\right)\left(c^2+ac\right)}\ge\sum\left(bc+a\sqrt{bc}\right)=abc+\sum a\sqrt{bc}\)
Dấu = xảy ra khi a=b=c=3
+) Giải hệ pt: \(\left\{{}\begin{matrix}4\sqrt{x^2+4y-5}=y^2-x+10\\x^3+\left(1-y\right)x^2=\left(x+4\right)y\end{matrix}\right.\)
+) Cho a,b,c>0 và a+b+c=2017
CM: \(\dfrac{2017a-a^2}{bc}+\dfrac{2017b-b^2}{ca}+\dfrac{2017c-c^2}{ab}\ge\sqrt{2}\left(\Sigma\sqrt{\dfrac{2017-a}{a}}\right)\)
+) Bài bất đẳng thức:
\(\dfrac{2017a-a^2}{bc}=\dfrac{\left(a+b+c\right)a-a^2}{bc}=\dfrac{ab+ca}{bc}=\dfrac{a}{c}+\dfrac{a}{b}\left(1\right)\)
Tương tự: \(\left\{{}\begin{matrix}\dfrac{2017b-b^2}{ca}=\dfrac{b}{a}+\dfrac{b}{c}\left(2\right)\\\dfrac{2017c-c^2}{ab}=\dfrac{c}{a}+\dfrac{c}{b}\left(3\right)\end{matrix}\right.\)
\(\left(1\right)+\left(2\right)+\left(3\right)\Rightarrow\dfrac{2017a-a^2}{bc}+\dfrac{2017b-b^2}{bc}+\dfrac{2017c-c^2}{ab}=\dfrac{a+b}{c}+\dfrac{b+c}{a}+\dfrac{c+a}{b}\)
\(\sqrt{2}\left(\sum\sqrt{\dfrac{2017-a}{a}}\right)=\sqrt{2}\left(\sum\sqrt{\dfrac{\left(a+b+c\right)-a}{a}}\right)=\sqrt{2}\left(\sqrt{\dfrac{b+c}{a}}+\sqrt{\dfrac{c+a}{b}}+\sqrt{\dfrac{a+b}{2}}\right)\)
Bất đẳng thức cần chứng minh tương đương với:
\(\dfrac{a+b}{c}+\dfrac{b+c}{a}+\dfrac{c+a}{b}\ge\sqrt{2}\left(\sqrt{\dfrac{a+b}{c}}+\sqrt{\dfrac{b+c}{a}}+\sqrt{\dfrac{c+a}{b}}\right)\)
*Có: \(\sqrt{2.\dfrac{a+b}{c}}+\sqrt{2.\dfrac{b+c}{a}}+\sqrt{2.\dfrac{c+a}{b}}\le\dfrac{2+\dfrac{a+b}{c}}{2}+\dfrac{2+\dfrac{b+c}{a}}{2}+\dfrac{2+\dfrac{c+a}{b}}{2}=3+\dfrac{\dfrac{a+b}{c}+\dfrac{b+c}{a}+\dfrac{c+a}{b}}{2}\)
Ta chỉ cần chứng minh:
\(\dfrac{a+b}{c}+\dfrac{b+c}{a}+\dfrac{c+a}{b}\ge3+\dfrac{\dfrac{a+b}{c}+\dfrac{b+c}{a}+\dfrac{c+a}{b}}{2}\)
hay \(\dfrac{a+b}{c}+\dfrac{b+c}{a}+\dfrac{c+a}{b}\ge6\) (cái này chị tự chứng minh nhé)
Anh Trần Tuấn Hoàng giỏi BĐT quá nhỉ
Tìm các số nguyên a,b,c
\(\left\{{}\begin{matrix}\dfrac{\sqrt{ab}-1}{3}=\dfrac{\sqrt{bc}-3}{9}=\dfrac{\sqrt{ac}-5}{-6}\\\sqrt{ab}+\sqrt{bc}+\sqrt{ac}=11\end{matrix}\right.\)
giải các hệ phương trình
a)\(\left\{{}\begin{matrix}-x+2y=6\\5x-3y=5\end{matrix}\right.\)
b)\(\left\{{}\begin{matrix}\dfrac{1}{3}x+\dfrac{1}{4}y-2=0\\5x-y=11\end{matrix}\right.\)
c)\(\left\{{}\begin{matrix}2\sqrt{3x}-\sqrt{5y}=2\sqrt{6}-\sqrt{15}\\3x-y=3\sqrt{2}-\sqrt{3}\end{matrix}\right.\)
a,\(\left\{{}\begin{matrix}-x+2y=6\\5x-3y=5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-3x+6y=18\left(1\right)\\10x-6y=10\left(2\right)\end{matrix}\right.\)
Cộng (1) và (2) => 7x=28
\(\Leftrightarrow\) x=4
thay x vào (1) ta có -4+2y=6
=> 2y=10
=>y=5
Vậy nghiệm của phương trình (x;y)=(4;5)
b: \(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{1}{3}x+\dfrac{1}{4}y=2\\5x-y=11\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=3\\y=4\end{matrix}\right.\)
c: \(\Leftrightarrow\left\{{}\begin{matrix}3x=6\\5y=15\\3x-y=3\sqrt{2}-\sqrt{3}\end{matrix}\right.\Leftrightarrow\left(x,y\right)\in\varnothing\)