Rút gọn : B=(1-\(\frac{1}{2}\)) x ( 1-\(\frac{1}{3}\)) x ( 1-\(\frac{1}{4}\)) x ..... x (1 - \(\frac{1}{20}\))
m.n giúp mk nha mk cần gấp
K
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26 tháng 1 2017
\(\frac{2.\left(x+4\right)}{\left(\sqrt{x}+1\right).\left(\sqrt{x}-4\right)}+\frac{\sqrt{x}.\left(\sqrt{x}-4\right)}{\left(\sqrt{x}+1\right).\left(\sqrt{x}-4\right)}-\frac{8.\left(\sqrt{x}+1\right)}{\left(\sqrt{x}-4\right)\left(\sqrt{x}+1\right)}\)
=\(\frac{2x+8+x-4\sqrt{x}-8\sqrt{x}-8}{\left(\sqrt{x}+1\right).\left(\sqrt{x}-4\right)}\)
=\(\frac{3x-12\sqrt{x}}{mc}\)
=\(\frac{3\sqrt{x}.\left(\sqrt{x}-4\right)}{\left(\sqrt{x-4}\right)\left(\sqrt{x}+1\right)}=\frac{3\sqrt{x}}{\sqrt{x}+1}\)
k tk mk cung lam cho
18 tháng 7 2017
\(=\frac{1}{\sqrt{x}\left(x\sqrt{x}-1\right)}:\frac{\sqrt{x}+1}{\sqrt{x}\left(x+\sqrt{x}+1\right)}\)
\(=\frac{1}{\sqrt{x}\left(\sqrt{x^3}-1\right)}.\frac{\sqrt{x}\left(x+\sqrt{x}+1\right)}{\sqrt{x}+1}\)
\(=\frac{1}{\sqrt{x}\left(\sqrt{x}-1\right)\left(x+\sqrt{x}+1\right)}.\frac{\sqrt{x}\left(x+\sqrt{x}+1\right)}{\sqrt{x}+1}\)
\(=\frac{1}{\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)}=\frac{1}{x-1}\)
TT
3
21 tháng 11 2017
Cái này dễ mà bn
Ta có:\(A=\frac{x+2}{x+3}-\frac{5}{x^2+x-6}+\frac{1}{2-x}\left(ĐK:x\ne2;-3\right)\)
\(\Leftrightarrow A=\frac{x+2}{x+3}-\frac{5}{\left(x+3\right)\left(x-2\right)}-\frac{1}{x-2}\)
\(\Leftrightarrow A=\frac{x^2-4-5-x-3}{\left(x+3\right)\left(x-2\right)}\)
\(\Leftrightarrow A=\frac{x^2-x-12}{\left(x+3\right)\left(x-2\right)}=\frac{x+4}{x-2}\)
NN
18 tháng 10 2020
a) \(ĐKXĐ:\hept{\begin{cases}x\ge0\\x\ne9\end{cases}}\)
\(M=\frac{2\sqrt{x}}{\sqrt{x}+3}+\frac{\sqrt{x}+1}{\sqrt{x}-3}+\frac{3-11\sqrt{x}}{9-x}\)
\(=\frac{2\sqrt{x}\left(\sqrt{x}-3\right)}{\left(\sqrt{x}+3\right)\left(\sqrt{x}-3\right)}+\frac{\left(\sqrt{x}+1\right)\left(\sqrt{x}+3\right)}{\left(\sqrt{x}+3\right)\left(\sqrt{x}-3\right)}+\frac{11\sqrt{x}-3}{x-9}\)
\(=\frac{2x-6\sqrt{x}}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}+\frac{x+4\sqrt{x}+3}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}+\frac{11\sqrt{x}-3}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\)
\(=\frac{2x-6\sqrt{x}+x+4\sqrt{x}+3+11\sqrt{x}-3}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\)
\(=\frac{3x+9\sqrt{x}}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}=\frac{3\sqrt{x}.\left(\sqrt{x}+3\right)}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}=\frac{3\sqrt{x}}{\sqrt{x}-3}\)
b) Ta có: \(x=\sqrt{\sqrt{3}-\sqrt{4-2\sqrt{3}}}=\sqrt{\sqrt{3}-\sqrt{3-2\sqrt{3}+1}}\)
\(=\sqrt{\sqrt{3}-\sqrt{\left(\sqrt{3}-1\right)^2}}=\sqrt{\sqrt{3}-\left|\sqrt{3}-1\right|}\)
\(=\sqrt{\sqrt{3}-\sqrt{3}+1}=\sqrt{1}=1\)( thỏa mãn ĐKXĐ )
Thay \(x=1\)vào M ta được:
\(M=\frac{3\sqrt{1}}{\sqrt{1}-3}=\frac{3}{1-3}=\frac{-3}{2}\)
c) \(M=\frac{3\sqrt{x}}{\sqrt{x}-3}=\frac{3\sqrt{x}-9+9}{\sqrt{x}-3}=\frac{3\left(\sqrt{x}-3\right)+9}{\sqrt{x}-3}=3+\frac{9}{\sqrt{x}-3}\)
Vì \(x\inℕ\)\(\Rightarrow\)Để M là số tự nhiên thì \(\frac{9}{\sqrt{x}-3}\inℕ\)
\(\Rightarrow9⋮\left(\sqrt{x}-3\right)\)\(\Rightarrow\sqrt{x}-3\inƯ\left(9\right)\)(1)
Vì \(x\ge0\)\(\Rightarrow\sqrt{x}\ge0\)\(\Rightarrow\sqrt{x}-3\ge-3\)(2)
Từ (1) và (2) \(\Rightarrow\sqrt{x}-3\in\left\{-3;-1;1;3;9\right\}\)
\(\Rightarrow\sqrt{x}\in\left\{0;2;4;6;12\right\}\)\(\Rightarrow x\in\left\{0;4;16;36;144\right\}\)( thỏa mãn ĐKXĐ )
Thử lại với \(x=4\)ta thấy M không là số tự nhiên
Vậy \(x\in\left\{0;16;36;144\right\}\)
KT
20 tháng 8 2018
a) ĐKXĐ: \(x\ne1\)
b) \(A=\frac{2}{x-1}+\frac{2\left(x+1\right)}{x^2+x+1}+\frac{x^2-10x+3}{x^3-1}\)
\(=\frac{2\left(x^2+x+1\right)}{\left(x-1\right)\left(x^2+x+1\right)}+\frac{2\left(x+1\right)\left(x-1\right)}{\left(x-1\right)\left(x^2+x+1\right)}+\frac{x^2-10x+3}{\left(x-1\right)\left(x^2+x+1\right)}\)
\(=\frac{2x^2+2x+2}{\left(x-1\right)\left(x^2+x+1\right)}+\frac{2x^2-2}{\left(x-1\right)\left(x^2+x+1\right)}+\frac{x^2-10x+3}{\left(x-1\right)\left(x^2+x+1\right)}\)
\(=\frac{5x^2-8x+3}{\left(x-1\right)\left(x^2+x+1\right)}=\frac{\left(x-1\right)\left(5x-3\right)}{\left(x-1\right)\left(x^2+x+1\right)}=\frac{5x-3}{x^2+x+1}\)
2 tháng 2 2018
Ta có :
\(\frac{x+1}{3}=\frac{-1}{y-2}\)\(\Rightarrow\)\(\left(x+1\right)\left(y-2\right)=\left(-1\right).3\)
\(\left(x+1\right)\left(y-2\right)=-3\)
TRƯỜNG HỢP 1 :
\(\hept{\begin{cases}x+1=1\\y-2=3\end{cases}\Leftrightarrow\hept{\begin{cases}x=0\\y=5\end{cases}}}\)
TRƯỜNG HỢP 2 :
\(\hept{\begin{cases}x+1=-1\\y-2=-3\end{cases}\Leftrightarrow\hept{\begin{cases}x=-2\\y=-1\end{cases}}}\)
TRƯỜNG HỢP 3 :
\(\hept{\begin{cases}x+1=3\\y-2=1\end{cases}\Leftrightarrow\hept{\begin{cases}x=2\\y=3\end{cases}}}\)
TRƯỜNG HỢP 4 :
\(\hept{\begin{cases}x+1=-3\\y-2=-1\end{cases}\Leftrightarrow\hept{\begin{cases}x=-4\\y=1\end{cases}}}\)
Vậy ...
\(B=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}....\frac{19}{20}=\frac{1}{20}\)
\(B=\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)\left(1-\frac{1}{4}\right)...\left(1-\frac{1}{20}\right)\)
\(B=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}...\frac{19}{20}\)
\(B=\frac{1}{20}\)