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\(\dfrac{1}{\sqrt{k}+\sqrt{k+1}}=\dfrac{\sqrt{k}-\sqrt{k+1}}{k-k-1}=\sqrt{k+1}-\sqrt{k}\\ \Leftrightarrow\text{Đặt}\text{ }A=\dfrac{1}{3\left(\sqrt{2}+\sqrt{1}\right)}+\dfrac{1}{5\left(\sqrt{3}+\sqrt{2}\right)}+...+\dfrac{1}{4021\left(\sqrt{2011}+\sqrt{2010}\right)}< \dfrac{1}{2\left(\sqrt{2}+\sqrt{1}\right)}+\dfrac{1}{2\left(\sqrt{3}+\sqrt{2}\right)}+...+\dfrac{1}{2\left(\sqrt{2011}+\sqrt{2010}\right)}\\ \Leftrightarrow A< \dfrac{1}{2}\left(\dfrac{1}{\sqrt{2}+\sqrt{1}}+\dfrac{1}{\sqrt{3}+\sqrt{2}}+...+\dfrac{1}{\sqrt{2011}+\sqrt{2010}}\right)\)
\(\Leftrightarrow A< \dfrac{1}{2}\left(\sqrt{2}-\sqrt{1}+\sqrt{3}-\sqrt{2}+...+\sqrt{2011}-\sqrt{2010}\right)\\ \Leftrightarrow A< \dfrac{1}{2}\left(\sqrt{2011}-1\right)< \dfrac{1}{2}\cdot\dfrac{\sqrt{2011}-1}{\sqrt{2011}}=\dfrac{1}{2}\left(1-\dfrac{1}{\sqrt{2011}}\right)\)
\(\frac{1}{2011}.x=\left(1-\frac{1}{2}\right).\left(1-\frac{1}{3}\right).\left(1-\frac{1}{4}\right)....\left(1-\frac{1}{2010}\right).\left(1-\frac{1}{2011}\right)\)
\(\frac{1}{2011}.x=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}....\frac{2009}{2010}.\frac{2010}{2011}\)
\(\frac{1}{2011}.x=\frac{1.2.3...2009.2010}{2.3.4...2010.2011}\)\(=\frac{1}{2011}\)
\(x=\frac{1}{2011}:\frac{1}{2011}=1\)
Vậy x=1
\(\frac{1}{2011}.x=\frac{1}{2}.\left(\frac{2}{3}\right).\left(\frac{3}{4}\right)......\left(\frac{2010}{2011}\right)\)
\(\frac{1}{2011}.x=\frac{2}{4}.\left(\frac{4}{6}\right).\left(\frac{6}{8}\right).......\left(\frac{4018}{4020}\right).\left(\frac{4020}{4022}\right)\)
\(\frac{1}{2011}.x=\frac{2.4.6.8.....4018.4020}{4.6.8.10.....4020.4022}\)
\(\frac{1}{2011}.x=\frac{2}{4022}\)
\(\Rightarrow\)\(x=\frac{2}{4022}:\frac{1}{2011}=1\)
Ai thấy đún thì ủng hộ mink nha !!!
Thanks you very much !!
Chúc các bạn luôn học giỏi !!!
\(a)\left(5^{2010}+5^{2012}+5^{2014}\right):\left(5^{2011}+5^{2009}+5^{2007}\right)\)
\(=\dfrac{5^{2007}\left(5^3+5^5+5^7\right)}{5^{2007}\left(5^4+5^2+1\right)}=\dfrac{5^3+5^5+5^7}{5^4+5^2+1}\)
\(=\dfrac{125+3125+78125}{625+25+1}=\dfrac{81375}{651}=125\)
\(b)-\dfrac{7}{45}+\dfrac{1}{4}+\dfrac{3}{5}+\dfrac{1}{12}+\dfrac{2}{3}+\dfrac{1}{39}+\dfrac{5}{9}\)
\(=\dfrac{-7.52+1.585+3.468+1.195+2.780+1.60-5.260}{2340}\)
\(=\dfrac{-364+585+1404+195+1560+60-1300}{2340}\)
\(=\dfrac{2140}{2340}=\dfrac{107}{117}\)
a) \(x^3-6x^2-9x+14=0\)
\(\Leftrightarrow x^3-8x^2+2x^2+7x-16x+14=0\)
\(\Leftrightarrow\left(x^3-8x^2+7x\right)+\left(2x^2-16x+14\right)=0\)
\(\Leftrightarrow x\left(x^2-8x+7\right)+2\left(x^2-8x+7\right)=0\)
\(\Leftrightarrow\left(x+2\right)\left(x^2-8x+7\right)=0\)
\(\Leftrightarrow\left(x+2\right)\left(x^2-7x-x+7\right)=0\)
\(\Leftrightarrow\left(x+2\right)\left[x\left(x-7\right)-\left(x-7\right)\right]=0\)
\(\Leftrightarrow\left(x+2\right)\left(x-1\right)\left(x-7\right)=0\)
\(\Leftrightarrow x\in\left\{-2;1;7\right\}\)
Lời giải:
a)
$x^3-6x^2-9x+14=0$
$\Leftrightarrow x^3-x^2-5x^2+5x-14x+14=0$
$\Leftrightarrow x^2(x-1)-5x(x-1)-14(x-1)=0$
$\Leftrightarrow (x-1)(x^2-5x-14)=0$
$\Leftrightarrow (x-1)(x^2-7x+2x-14)=0$
$\Leftrightarrow (x-1)[x(x-7)+2(x-7)]=0$
$\Leftrightarrow (x-1)(x+2)(x-7)=0$
$\Rightarrow x=1; x=-2$ hoặc $x=7$
b)
Bạn tham khảo tại đây:
Câu hỏi của Lương Đức Hưng - Toán lớp 8 | Học trực tuyến
Vì số mũ lẻ
=> (-1). (-1)2. (-1)3. (-1)4.... (-1)2011= -1
thì làm sao???Hỏi xong rồi tự trả lời thì có ích gì
(✿◠‿◠)(๛ČℌUƔÊŇ♥Ť❍Ą́Ňツ)
Ê nhóc đừng có nghĩ lung tung