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Phân tích phân số \(\dfrac{30}{43}\) ta có:
\(\dfrac{30}{43}=\dfrac{1}{\dfrac{43}{30}}=\dfrac{1}{1+\dfrac{13}{30}}=\dfrac{1}{1+\dfrac{1}{\dfrac{30}{13}}}=\dfrac{1}{1+\dfrac{1}{2+\dfrac{4}{13}}}\)
\(=\dfrac{1}{1+\dfrac{1}{2+\dfrac{1}{\dfrac{13}{4}}}}=\dfrac{1}{1+\dfrac{1}{2+\dfrac{1}{3+\dfrac{1}{4}}}}=\dfrac{1}{a+\dfrac{1}{b+\dfrac{1}{c+\dfrac{1}{d}}}}\)
Vậy: \(\left\{{}\begin{matrix}a=1\\b=2\\c=3\\d=4\end{matrix}\right.\)
Có \(\dfrac{30}{43}=\dfrac{1}{\dfrac{43}{30}}=\dfrac{1}{1+\dfrac{13}{30}}=\dfrac{1}{1+\dfrac{1}{\dfrac{30}{13}}}=\dfrac{1}{1+\dfrac{1}{2+\dfrac{4}{13}}}=\dfrac{1}{1+\dfrac{1}{2+\dfrac{1}{\dfrac{13}{4}}}}=\dfrac{1}{1+\dfrac{1}{2+\dfrac{1}{3+\dfrac{1}{4}}}}\)
Vậy a=1; b=2 ; c=3 ; d=4
ta thấy : \(\dfrac{a}{b}\) = \(\dfrac{1}{\dfrac{b}{a}}\)
\(\Rightarrow\) \(\dfrac{30}{43}\) = \(\dfrac{1}{\dfrac{43}{30}}\)
= \(\dfrac{1}{1+\dfrac{13}{30}}\)
= \(\dfrac{1}{1+\dfrac{1}{\dfrac{30}{13}}}\)
= \(\dfrac{1}{1+\dfrac{1}{2+\dfrac{2}{15}}}\)
= \(\dfrac{1}{1+\dfrac{1}{2+\dfrac{1}{\dfrac{15}{2}}}}\)
=\(\dfrac{1}{1+\dfrac{1}{2+\dfrac{1}{7+\dfrac{1}{2}}}}\)
Vậy a = 1; b = 2; c = 7; d = 4
\(\dfrac{30}{43}=\dfrac{1}{a+\dfrac{1}{b+\dfrac{1}{c+\dfrac{1}{d}}}}\)
\(\Leftrightarrow\dfrac{1}{\dfrac{43}{30}}=\dfrac{1}{a+\dfrac{1}{b+\dfrac{1}{c+\dfrac{1}{d}}}}\\ \Leftrightarrow\dfrac{1}{1+\dfrac{13}{30}}=\dfrac{1}{a+\dfrac{1}{b+\dfrac{1}{c+\dfrac{1}{d}}}}\\ \Leftrightarrow\dfrac{1}{1+\dfrac{1}{\dfrac{30}{13}}}=\dfrac{1}{a+\dfrac{1}{b+\dfrac{1}{c+\dfrac{1}{d}}}}\\ \Leftrightarrow\dfrac{1}{1+\dfrac{1}{2+\dfrac{4}{13}}}=\dfrac{1}{a+\dfrac{1}{b+\dfrac{1}{c+\dfrac{1}{d}}}}\)
\(\\ \Leftrightarrow\dfrac{1}{1+\dfrac{1}{2+\dfrac{1}{\dfrac{13}{4}}}}=\dfrac{1}{a+\dfrac{1}{b+\dfrac{1}{c+\dfrac{1}{d}}}}\\\Leftrightarrow\dfrac{1}{1+\dfrac{1}{2+\dfrac{1}{3+\dfrac{1}{4}}}}=\dfrac{1}{a+\dfrac{1}{b+\dfrac{1}{c+\dfrac{1}{d}}}}\)
\(\Rightarrow\left\{{}\begin{matrix}a=1\\b=2\\c=3\\d=4\end{matrix}\right.\)
Vậy............
Bài 2:
a) Ta có: \(A=\dfrac{4}{n-1}+\dfrac{6}{n-1}-\dfrac{3}{n-1}\)
\(=\dfrac{4+6-3}{n-1}\)
\(=\dfrac{7}{n-1}\)
Để A là số tự nhiên thì \(7⋮n-1\)
\(\Leftrightarrow n-1\inƯ\left(7\right)\)
\(\Leftrightarrow n-1\in\left\{1;7\right\}\)
hay \(n\in\left\{2;8\right\}\)
Vậy: \(n\in\left\{2;8\right\}\)
ta có B=2n+9/n+2-3n+5n+1/n+2=4n+10/n+2 Để B là STN thì 4n+10⋮n+2 4n+8+2⋮n+2 4n+8⋮n+2 ⇒2⋮n+2 n+2∈Ư(2) Ư(2)={1;2} Vậy n=0
a, \(\dfrac{x}{2}=-\dfrac{5}{y}\Rightarrow xy=-10\Rightarrow x;y\inƯ\left(-10\right)=\left\{\pm1;\pm2;\pm5;\pm10\right\}\)
| x | 1 | -1 | 2 | -2 | 5 | -5 | 10 | -10 |
| y | -10 | 10 | -5 | 5 | -2 | 2 | -1 | 1 |
c, \(\dfrac{3}{x-1}=y+1\Rightarrow\left(y+1\right)\left(x-1\right)=3\Rightarrow x-1;y+1\inƯ\left(3\right)=\left\{\pm1;\pm3\right\}\)
| x - 1 | 1 | -1 | 3 | -3 |
| y + 1 | 3 | -3 | 1 | -1 |
| x | 2 | 0 | 4 | -2 |
| y | 2 | -4 | 0 | -2 |
b: =>xy=12
\(\Leftrightarrow\left(x,y\right)\in\left\{\left(12;1\right);\left(6;2\right);\left(4;3\right)\right\}\)
\(\dfrac{1}{a}-\dfrac{1}{b}=1\)
\(\Leftrightarrow\dfrac{b-a}{ab}=1\)
\(\Leftrightarrow b-a=ab\)
\(\Leftrightarrow a+ab-b=0\)
a)4/5+x=2/3
x=2/3-4/5
x=-2/15
b)-5/6-x=2/3
x=-5/6-2/3
x=-3/2
c)1/2x+3/4=-3/10
1/2x=-3/10-3/4
1/2x=-21/20
x=-21/20:1/2
x=-21/10
d)x/3-1/2=1/5
x/3=1/5+1/2
x/3=7/10
10x/30=21/30
10x=21
x=21:10
x=21/10
a) \(\dfrac{n+2}{3}\) là số tự nhiên khi
\(n+2⋮3\)
\(\Rightarrow n+2\in\left\{1;3\right\}\)
\(\Rightarrow n\in\left\{-1;1\right\}\left(n\in Z\right)\)
b) \(\dfrac{7}{n-1}\) là số tự nhiên khi
\(7⋮n-1\)
\(\Rightarrow7n-7\left(n-1\right)⋮n-1\)
\(\Rightarrow7n-7n+7⋮n-1\)
\(\Rightarrow7⋮n-1\)
\(\Rightarrow n-1\in\left\{1;7\right\}\Rightarrow\Rightarrow n\in\left\{2;8\right\}\left(n\in Z\right)\)
c) \(\dfrac{n+1}{n-1}\) là sô tự nhiên khi
\(n+1⋮n-1\)
\(\Rightarrow n+1-\left(n-1\right)⋮n-1\)
\(\Rightarrow n+1-n+1⋮n-1\)
\(\Rightarrow2⋮n-1\)
\(\Rightarrow n-1\in\left\{1;2\right\}\Rightarrow n\in\left\{2;3\right\}\left(n\in Z\right)\)
