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Câu hỏi của Trần Anh Thơ - Toán lớp 8 | Học trực tuyến
Ko phải số thực dương thì hơi mất thời gian
Ta có: \(\left|x\right|+\left|y\right|\ge x+y\Rightarrow\left|x\right|+\left|y\right|+\left|z\right|\ge x+y+z\)
\(\Rightarrow\frac{bc}{a^2+1}+\frac{ca}{b^2+1}+\frac{ab}{c^2+1}\le\frac{\left|bc\right|}{a^2+1}+\frac{\left|ca\right|}{b^2+1}+\frac{\left|ab\right|}{c^2+1}\)
Đặt \(\left(\left|a\right|;\left|b\right|;\left|c\right|\right)=\left(x;y;z\right)\Rightarrow x;y;z\ge0\)
\(\Rightarrow VT\le\frac{yz}{x^2+1}+\frac{zx}{y^2+1}+\frac{xy}{z^2+1}=\frac{yz}{x^2+y^2+x^2+z^2}+\frac{zx}{x^2+y^2+y^2+z^2}+\frac{xy}{x^2+z^2+y^2+z^2}\)
\(VT\le\frac{yz}{2\sqrt{\left(x^2+y^2\right)\left(x^2+z^2\right)}}+\frac{zx}{2\sqrt{\left(x^2+y^2\right)\left(y^2+z^2\right)}}+\frac{xy}{2\sqrt{\left(x^2+z^2\right)\left(y^2+z^2\right)}}\)
\(VT\le\frac{1}{4}\left(\frac{y^2}{x^2+y^2}+\frac{z^2}{x^2+z^2}+\frac{x^2}{x^2+y^2}+\frac{z^2}{y^2+z^2}+\frac{x^2}{x^2+z^2}+\frac{y^2}{y^2+z^2}\right)=\frac{3}{4}\)
Dấu "=" xảy ra khi \(x=y=z=\frac{1}{\sqrt{3}}\) hay \(a=b=c=\pm\frac{1}{\sqrt{3}}\)
Áp dụng BĐT: \(\left(x+y+z\right)\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right)\ge9\) ( Câu hỏi của ZoZ - Kudo vs Conan - ZoZ - Toán lớp 9 | Học trực tuyến)
\(\Rightarrow\dfrac{1}{x+y+z}\le\dfrac{1}{9}\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right)\)
Áp dụng vào, ta có:
\(\dfrac{ab}{a+3b+2c}=\dfrac{ab}{\left(a+c\right)+\left(b+c\right)+2b}\le\dfrac{ab}{9}\left(\dfrac{1}{a+c}+\dfrac{1}{b+c}+\dfrac{1}{2b}\right)\)\(\dfrac{bc}{b+3c+2a}=\dfrac{bc}{\left(a+c\right)+\left(a+b\right)+2c}\le\dfrac{9}{bc}\left(\dfrac{1}{a+c}+\dfrac{1}{a+b}+\dfrac{1}{2c}\right)\)\(\dfrac{ca}{c+3a+2b}=\dfrac{ca}{\left(c+b\right)+\left(b+a\right)+2a}\le\dfrac{ca}{9}\left(\dfrac{1}{c+b}+\dfrac{1}{b+a}+\dfrac{1}{2a}\right)\)
Cộng vế theo vế BĐT, ta được:
\(P\le\dfrac{1}{9}\left(\dfrac{bc+ac}{a+b}+\dfrac{bc+ab}{a+c}+\dfrac{ab+ac}{b+c}\right)+\dfrac{1}{18}\left(a+b+c\right)\)
\(P\le\dfrac{1}{9}\left[\dfrac{c\left(a+b\right)}{a+b}+\dfrac{b\left(c+a\right)}{a+c}+\dfrac{a\left(b+c\right)}{b+c}\right]+\dfrac{1}{18}\left(a+b+c\right)\)
\(P\le\dfrac{1}{9}\left(a+b+c\right)+\dfrac{1}{18}\left(a+b+c\right)\)
\(P\le\dfrac{1}{6}\left(a+b+c\right)\) \(=\dfrac{1}{6}.6=1\)
\(\Rightarrow Max_P=1\Leftrightarrow a=b=c=2\)
a/
\(a^2+b^2+c^2+29ab+bc+ca=3\left(ab+bc+ca\right)\)
\(\Leftrightarrow a^2+b^2+c^2-ab-bc-ca=0\)
\(\Leftrightarrow2a^2+2b^2+2c^2-2ab-2bc-2ca=0\)
\(\Leftrightarrow\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}a-b=0\\b-c=0\\c-a=0\end{matrix}\right.\) \(\Rightarrow a=b=c\)
b/ \(a^3+b^3+c^3=\left(a+b\right)^3+c^3-3ab\left(a+b\right)\)
\(=\left(a+b+c\right)\left(\left(a+b\right)^2-c\left(a+b\right)+c^2\right)-3ab\left(a+b\right)\)
\(=-3ab\left(a+b\right)=-3ab\left(-c\right)=3abc\)
c/ Không, vì \(a=b=c\ne\) thì \(a^3+b^3+c^3=3a^3=3abc\) vẫn đúng
Ta có
\(x^2+y^2\ge2xy\)hay\(xy\le\frac{x^2+y^2}{2}\left(\forall x,y\right)\)
\(=>ab+bc+ca+a+b+c\le\frac{a^2+b^2}{2}+\frac{b^2+c^2}{2}+\frac{c^2+a^2}{2}+\frac{a^2+1}{2}\)
\(+\frac{b^2+1}{2}+\frac{c^2+1}{2}\)
\(=a^2+b^2+c^2+\frac{a^2+b^2+c^2+3}{2}\left(do\right)a^2+b^2+c^2=3\)
\(=>=3+\frac{3+3}{2}=6\)
=> dpcm
cậu zô trang tuyển tập những toán hay nhá. Nơi đó nhiều bài hay lắm
(a - b)^2 = a^2 - 2ab + b^2 > 0
(b - c)^2 = b^2 - 2bc + c^2 > 0
(c - a)^2 = c^2 - 2ac + a^2 > 0
=> 2a^2 + 2b^2 + 2c^2 > 2ab + 2bc + 2ac
=> 6 > 2ab + 2bc + 2ac
=> 3 > ab + bc + ac (1)
(a - 1)^2 = a^2 - 2a + 1 > 0
(b - 1)^2 = b^2 - 2b + 1 > 0
(c - 1)^2 = c^2 - 2c + 1 > 0
=> a^2 + b^2 + c^2 + 1 + 1 + 1 > 2a + 2b + 2c
=> 6 > 2a + 2b + 2c
=> 3 > a + b + c và (1)
=> 6 > ab + ac + bc + a + b + c
1. ĐKXĐ: ...
Đặt \(2\sqrt{x+2}+\sqrt{4x+1}=t\ge\sqrt{7}\)
\(\Rightarrow t^2=8x+9+4\sqrt{4x^2+9x+2}\)
\(\Rightarrow2x+\sqrt{4x^2+9x+2}=\frac{t^2-9}{4}\)
Phương trình trở thành:
\(\frac{t^2-9}{4}+3=t\)
\(\Leftrightarrow t^2-4t+3=0\Rightarrow\left[{}\begin{matrix}t=1\left(l\right)\\t=3\end{matrix}\right.\)
\(\Rightarrow4\sqrt{4x^2+9x+2}=t^2-\left(8x+9\right)=-8x\) (\(x\le0\))
\(\Leftrightarrow\sqrt{4x^2+9x+2}=-2x\)
\(\Leftrightarrow4x^2+9x+2=4x^2\Rightarrow x=-\frac{2}{9}\)
Bài 2:
Ta có: \(a+b+c\le\sqrt{3\left(a^2+b^2+c^2\right)}\Rightarrow3\ge a+b+c\)
Do \(\frac{a}{\sqrt{b}}+\frac{b}{\sqrt{c}}+\frac{c}{\sqrt{a}}\ge\frac{\left(\sqrt{a}+\sqrt{b}+\sqrt{c}\right)^2}{\sqrt{a}+\sqrt{b}+\sqrt{c}}=\sqrt{a}+\sqrt{b}+\sqrt{c}\)
Nên BĐT sẽ được chứng minh nếu ta chỉ ra rằng:
\(\sqrt{a}+\sqrt{b}+\sqrt{c}\ge ab+bc+ca\)
Thật vậy, ta có:
\(\sqrt{a}+\sqrt{a}+a^2\ge3a\) ; \(\sqrt{b}+\sqrt{b}+b^2\ge3b\) ; \(\sqrt{c}+\sqrt{c}+c^2\ge3c\)
\(\Rightarrow2\left(\sqrt{a}+\sqrt{b}+\sqrt{c}\right)+a^2+b^2+c^2\ge3\left(a+b+c\right)\ge\left(a+b+c\right)^2\)
\(\Leftrightarrow2\left(\sqrt{a}+\sqrt{b}+\sqrt{c}\right)+a^2+b^2+c^2\ge a^2+b^2+c^2+2\left(ab+bc+ca\right)\)
\(\Leftrightarrow\sqrt{a}+\sqrt{b}+\sqrt{c}\ge ab+bc+ca\) (đpcm)
Dấu "=" xảy ra khi \(a=b=c=1\)
\(A=\frac{bc}{a^2}+\frac{ac}{b^2}+\frac{ab}{c^2}=abc\left(\frac{1}{a^3}+\frac{1}{b^3}+\frac{1}{c^3}\right)\)
Ta có: \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=0\Rightarrow\frac{1}{a}+\frac{1}{b}=-\frac{1}{c}\)
\(\Rightarrow\left(\frac{1}{a}+\frac{1}{b}\right)^3=-\frac{1}{c^3}\Rightarrow\frac{1}{a^3}+\frac{1}{b^3}+\frac{1}{c^3}+\frac{3}{ab}\left(\frac{1}{a}+\frac{1}{b}\right)=0\)
\(\Rightarrow\frac{1}{a^3}+\frac{1}{b^3}+\frac{1}{c^3}+\frac{3}{ab}.\frac{-1}{c}\Rightarrow\frac{1}{a^3}+\frac{1}{b^3}+\frac{1}{c^3}=\frac{3}{abc}\)
\(\Rightarrow A=\frac{3}{abc}.abc=3\)
Với mọi số thực a;b;c ta luôn có:
\(\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2\ge0\)
\(\Leftrightarrow2a^2+2b^2+2c^2\ge2ab+2bc+2ca\) (1)
Tương tự: \(\left(a-1\right)^2+\left(b-1\right)^2+\left(c-1\right)^2\ge0\)
\(\Leftrightarrow a^2+b^2+c^2+3\ge2a+2b+2c\) (2)
Cộng vế với vế (1) và (2)
\(\Rightarrow3\left(a^2+b^2+c^2\right)+3\ge2\left(ab+bc+ca+a+b+c\right)\)
\(\Leftrightarrow ab+bc+ca+a+b+c\le6\)
Dấu "=" xảy ra khi \(a=b=c=1\)