A\(=\frac{x+2}{x-2}\)
Tìm số nguyên cuả x để A đạt giá trị nguyên
GIÚP MÌNH VỚI CHIỀU MINH ĐI HỌC RÔI
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T
30 tháng 11 2018
a) M xác định khi \(x+1\ne0\)
\(x^2+1\ne0\)
\(x^2+2x+1=\left(x+1\right)^2\ne0\)
\(\Leftrightarrow x\ne\pm1\)
b) \(M=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}\left(\frac{1}{x^2+2x+1}-\frac{1}{x^2-1}\right)\)
\(=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}\left(\frac{1}{\left(x+1\right)^2}-\frac{1}{\left(x-1\right)\left(x+1\right)}\right)\)
\(=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}\left(\frac{1\left(x-1\right)\left(x+1\right)}{\left(x+1\right)^2\left(x-1\right)\left(x+1\right)}-\frac{1\left(x+1\right)^2}{\left(x+1\right)^2\left(x-1\right)\left(x+1\right)}\right)\)
\(=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}\left(\frac{\left[1\left(x^2-1\right)\right]-1\left(x+1\right)^2}{\left(x+1\right)^2\left(x-1\right)\left(x+1\right)}\right)\)
\(=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}.\frac{x^2-1-1\left(x^2+2x+1\right)}{\left(x+1\right)^2\left(x-1\right)\left(x+1\right)}\)
\(=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}.\frac{x^2-1-x^2-2x-1}{\left(x+1\right)^2\left(x-1\right)\left(x+1\right)}\)
\(=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}.\frac{-2x-2}{\left(x+1\right)^2\left(x-1\right)\left(x+1\right)}\)
\(=\frac{1}{x+1}+\frac{\left(x-x^3\right)\left(-2x-2\right)}{\left(x^2+1\right)\left(x^2-1\right)\left(x+1\right)^2}\)\(=\frac{1}{x+1}+\frac{\left(x-x^3\right)\left(-2x-2\right)}{\left(x^4-1\right)\left(x+1\right)^2}\)
\(=\frac{1}{x+1}+\frac{-2\left(x-x^3\right)\left(x+1\right)}{\left(x^4-1\right)\left(x+1\right)^2}\)\(=\frac{1}{x+1}+\frac{-2\left(x-x^3\right)}{\left(x^4-1\right)\left(x+1\right)}\)
\(=\frac{\left(x^4-1\right)\left(x+1\right)}{\left(x+1\right)\left(x^4-1\right)\left(x+1\right)}+\frac{-2\left(x-x^3\right)\left(x+1\right)}{\left(x^4-1\right)\left(x+1\right)}\)
\(=\frac{\left(x^4-1\right)}{\left(x+1\right)\left(x^4-1\right)}+\frac{-2\left(x-x^3\right)}{\left(x^4-1\right)}\)\(=\frac{1}{x+1}+\frac{-2\left(x-x^3\right)}{\left(x^4-1\right)}\)??? Chắc hết rút được rồi :v
T
30 tháng 11 2018
Câu b) hơi dài quá rồi.Làm lại
b) \(M=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}\left(\frac{1}{x^2+2x+1}-\frac{1}{x^2-1}\right)\)
\(=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}\left(\frac{1}{\left(x+1\right)^2}-\frac{1}{\left(x-1\right)\left(x+1\right)}\right)\)
\(=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}\left(\frac{x-1}{\left(x+1\right)^2\left(x-1\right)}-\frac{x+1}{\left(x+1\right)^2\left(x-1\right)}\right)\)
\(=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}\left(\frac{\left(x-1\right)-\left(x+1\right)}{\left(x+1\right)^2\left(x-1\right)}\right)\)\(=\frac{1}{x+1}+\frac{x-x^3}{x^2+1}.\frac{-2}{\left(x+1\right)^2\left(x-1\right)}\)
\(=\frac{1}{x+1}+\frac{-2\left(x-x^3\right)}{\left(x^2+1\right)\left(x+1\right)^2\left(x-1\right)}\)\(=\frac{1}{x+1}+\frac{2x\left(x+1\right)\left(x+1\right)}{\left(x^2+1\right)\left(x+1\right)^2\left(x-1\right)}\)
\(=\frac{1}{x+1}+\frac{2x}{\left(x^2+1\right)\left(x+1\right)}=\frac{x+1}{x^2+1}\) (Quy đồng và rút gọn)
LM
3
21 tháng 1 2022
A∈Z⇒\(\dfrac{2\left(x+1\right)}{x+3}\in Z\Rightarrow\left(2x+2\right)⋮\left(x+3\right)\)
\(\Rightarrow\left(2x+6-4\right)⋮\left(x+3\right)\\ \Rightarrow\left[2\left(x+3\right)-4\right]⋮\left(x+3\right)\)
\(\text{Mà}2\left(x+3\right)⋮\left(x+3\right)\\ \Rightarrow-4⋮\left(x+3\right)\\ \Rightarrow x+3\inƯ\left(-4\right)=\left\{-4;-2;-1;1;2;4\right\}\\ \Rightarrow x\in\left\{-7;-5;-4;-2;-1;1\right\}\)
S
4 tháng 12 2018
a, DKXD: \(x\ne\pm3\)
\(A=\left(\frac{x}{x+3}+\frac{x-1}{x-3}+\frac{2x^2+x-3}{9-x^2}\right):\frac{-2}{x-3}\)
\(=\left(\frac{x\left(x+3\right)+\left(x-1\right)\left(x+3\right)}{\left(x+3\right)\left(x-3\right)}+\frac{-2x^2-x+3}{x^2-9}\right):\frac{-2}{x-3}\)
\(=\left(\frac{2x^2+5x-3}{x^2-9}+\frac{-2x^2-x+3}{x^2-9}\right):\frac{-2}{x-3}\)
\(=\frac{4x}{x^2-9}:\frac{-2}{x-3}=\frac{4x}{\left(x-3\right)\left(x+3\right)}\cdot\frac{x-3}{-2}=\frac{4x}{-2\left(x+3\right)}=\frac{-2x}{x+3}\)
b, \(x^2-2x-3=0\Leftrightarrow x^2-3x+x-3=0\Leftrightarrow x\left(x-3\right)+\left(x-3\right)=0\)
\(\Leftrightarrow\left(x+1\right)\left(x-3\right)=0\Leftrightarrow\orbr{\begin{cases}x+1=0\\x-3=0\end{cases}\Leftrightarrow\orbr{\begin{cases}x=-1\\x=3\end{cases}}}\)
Thay x=-1 =>\(A=\frac{-2.\left(-1\right)}{-1+3}=1\)
thay x=3 =>\(A=\frac{-2.3}{3+3}=-1\)
c, De \(A\in Z\Leftrightarrow x+3\in U\left(-2\right)=\left\{1;-1;2;-2\right\}\)
<=>x thuoc {-2;-4;-1;-5}
4 tháng 12 2018
ĐK: \(x\ne\pm3\)
\(A=\left(\frac{x}{x+3}+\frac{x-1}{x-3}+\frac{2x^2+x-3}{9-x^2}\right):\frac{-2}{x-3}\)
\(=\left(\frac{x\left(x-3\right)+\left(x+3\right)\left(x-1\right)}{\left(x+3\right)\left(x-3\right)}+\frac{-2x^2-x+3}{x^2-9}\right).\frac{x-3}{-2}\)
\(=\left(\frac{x^2-3x+x^2+2x-3}{\left(x-3\right)\left(x+3\right)}+\frac{-2x^2-x+3}{\left(x-3\right)\left(x+3\right)}\right).\frac{x-3}{-2}\)
\(=\frac{-2x}{\left(x-3\right)\left(x+3\right)}.\frac{x-3}{-2}=\frac{x}{x+3}\)
b, \(x^2-2x-3=0\Rightarrow x\left(x-3\right)+\left(x-3\right)=0\Rightarrow\left(x-3\right)\left(x+1\right)=0\Rightarrow\orbr{\begin{cases}x=3\\x=-1\end{cases}}\)
x = 3 không thỏa mãn ĐKXĐ
Với x = -1 (thỏa mãn ĐKXĐ) thì \(A=\frac{x}{x+3}=\frac{-1}{-1+3}=-\frac{1}{2}\)
c, \(A\in Z\Rightarrow\frac{x}{x+3}\in Z\Rightarrow x⋮\left(x+3\right)\)
\(\Rightarrow\left(x+3\right)-3⋮\left(x+3\right)\Rightarrow-3⋮\left(x+3\right)\Rightarrow x+3\inƯ\left(-3\right)=\left\{-3;-1;1;3\right\}\)
\(\Rightarrow x\in\left\{-6;-4;-2;0\right\}\) (thỏa mãn điều kiện)
5 tháng 1 2022
Để (2x+2)/(x+3) là số nguyên thì \(x+3\in\left\{1;-1;2;-2;4;-4\right\}\)
hay \(x\in\left\{-2;-4;-1;-5;1;-7\right\}\)
5 tháng 1 2022
\(\dfrac{2x+2}{x+3}=\dfrac{2\left(x+3\right)-4}{x+3}=2-\dfrac{4}{x+3}\in Z\\ \Leftrightarrow x+3\inƯ\left(4\right)=\left\{-4;-2;-1;1;2;4\right\}\\ \Leftrightarrow x\in\left\{-7;-5;-4;-2;-1;1\right\}\)
VD
18 tháng 3 2022
\(a,\dfrac{x+1}{x-1}-\dfrac{x-1}{x+1}+\dfrac{4}{1-x^2}\\ =\dfrac{\left(x+1\right)^2}{\left(x-1\right)\left(x+1\right)}-\dfrac{\left(x-1\right)^2}{\left(x+1\right)\left(x-1\right)}-\dfrac{4}{\left(x-1\right)\left(x+1\right)}\\ =\dfrac{x^2+2x+1-x^2+2x-1-4}{\left(x-1\right)\left(x+1\right)}\\ =\dfrac{4x-4}{\left(x-1\right)\left(x+1\right)}\\ =\dfrac{4\left(x-1\right)}{\left(x-1\right)\left(x+1\right)}\\ =\dfrac{4}{x+1}\)
b, \(P=2022\)
\(\Leftrightarrow\dfrac{4}{x+1}=2022\\ \Leftrightarrow4=2022x+2022\\ \Leftrightarrow2022x=-2018\\ \Leftrightarrow x=-\dfrac{1009}{1011}\)
c, P nguyên
\(\Leftrightarrow\dfrac{4}{x+1}\in Z\\ \Rightarrow4⋮\left(x+1\right)\\ \Rightarrow x+1\inƯ\left(4\right)\)
Ta có bảng:
| x+1 | -4 | -2 | -1 | 1 | 2 | 4 |
| x | -5 | -3 | -2 | 0 | 1(loại) | 3 |
Vậy \(x\in\left\{-5;-3;-2;0;3\right\}\)
Ta có: \(A=\frac{x+2}{x-2}=\frac{x-2+4}{x-2}=1+\frac{4}{x-2}\)
Để A nguyên thì \(\frac{4}{x-2}\) nguyên hay \(x-2\inƯ\left(4\right)=\left\{\pm1;\pm2;\pm4\right\}\)
Đến đây lập bảng xét từng giá trị của x - 2 và tìm x. =))
Vì \(x\inℤ\Rightarrow x+2\inℤ;x-2\inℤ\)
\(\Rightarrow A\inℤ\Leftrightarrow\frac{x+2}{x-2}\inℤ\)
\(\Leftrightarrow x+2⋮x-2\)
\(\Leftrightarrow\left(x-2\right)+4⋮x-2\)
\(\Leftrightarrow4⋮x-2\left(x-2⋮x-2\right)\)
\(\Leftrightarrow x-2\inƯ\left(4\right)=\left\{\pm1;\pm2;\pm4\right\}\)
Ta có bảng sau :
Vậy \(x=-2;0;1;3;4;6\)