BCNN của (7;21;15) là: ?
Giúp mik với
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![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Trong 1 giờ hai vòi chảy được \(\frac{1}{2}\text{ bể}\), riêng vòi thứ nhất chảy được \(\frac{1}{3}\text{ bể}\).
thế nên trong 1 giờ vòi thứ hai chảy được thể tích bể là : \(\frac{1}{2}-\frac{1}{3}=\frac{1}{6}\text{ bể}\)
Hay vòi thứ hai chảy đầy bể trong 6 giờ
![](https://rs.olm.vn/images/avt/0.png?1311)
\(xy-y+x-4=0\)
\(\Leftrightarrow y.\left(x-1\right)+\left(x-1\right)-3=0\)
\(\Leftrightarrow\left(y+1\right).\left(x-1\right)=3\)
\(\Leftrightarrow\left(y+1\right);\left(x-1\right)\inƯ\left(3\right)\)
\(\Leftrightarrow\left(y+1\right);\left(x-1\right)\in\left\{-3;-1;1;3\right\}\)
Lập bảng:
y+1 | -3 | -1 | 1 | 3 |
y | -4 | -2 | 0 | 2 |
x-1 | -3 | -1 | 1 | 3 |
x | -2 | 0 | 3 | 4 |
Vậy \(\left(x;y\right)\in\left\{\left(-4;-2\right);\left(-2;0\right);\left(0;3\right);\left(2;4\right)\right\}\)
\(\Rightarrow\)xy-y+x=-4
\(\Leftrightarrow\left(xy-y\right)+x=-4\)
\(\Leftrightarrow y\left(x-1\right)+x=-4\)
\(\Leftrightarrow y\left(x-1\right)+x-1=-5\)
\(\Leftrightarrow\left(y+1\right)\left(x-1\right)=-5\)
lại có \(-5=-5.1=-1.5\)
\(\Rightarrow\left(y+1\right)\left(x-1\right)=1.\left(-5\right)=\left(-1\right).5\)
Xét (y+1)(x-1)=1.(-5)
+ TH1:\(\hept{\begin{cases}y+1=1\\x-1=-5\end{cases}}\)\(\Rightarrow\hept{\begin{cases}y=0\\x=-4\end{cases}}\)
+ TH2 \(\hept{\begin{cases}y+1=-5\\x-1=1\end{cases}}\Rightarrow\hept{\begin{cases}y=-6\\x=2\end{cases}}\)
Xét (y+1)(x+1)=-1.5(tự làm nhé bạn)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\left(\frac{1}{2}-1\right)\times\left(\frac{1}{3}-1\right)\times\left(\frac{1}{4}-1\right)\times...\times\left(\frac{1}{200}-1\right)\)
\(=-\left(1-\frac{1}{2}\right)\times\left(1-\frac{1}{3}\right)\times\left(1-\frac{1}{4}\right)\times...\times\left(1-\frac{1}{200}\right)\)
\(=-\frac{1}{2}\times\frac{2}{3}\times\frac{3}{4}\times...\times\frac{199}{200}\)
\(=-\frac{1}{200}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\left(-\dfrac{1}{4}+\dfrac{7}{33}-\dfrac{5}{3}\right)-\left(-\dfrac{15}{12}+\dfrac{6}{11}-\dfrac{48}{49}\right)\)
=\(\dfrac{1}{4}+\dfrac{7}{33}-\dfrac{5}{3}+\dfrac{15}{12}-\dfrac{6}{11}+\dfrac{48}{49}\)
=\(\left(-\dfrac{1}{4}-\dfrac{5}{3}+\dfrac{5}{12}\right)+\left(\dfrac{7}{33}-\dfrac{6}{11}\right)+\dfrac{48}{49}\)
\(=\left(-\dfrac{23}{12}+\dfrac{15}{12}\right)+\left(\dfrac{7}{33}-\dfrac{18}{33}\right)+\dfrac{48}{49}\)
=\(-\dfrac{2}{3}+\dfrac{-1}{3}+\dfrac{48}{49}\)
=\(-1+\dfrac{48}{49}=-\dfrac{49}{49}+\dfrac{48}{49}=-\dfrac{1}{49}\)
\(\left(\frac{-1}{4}+\frac{7}{33}-\frac{5}{3}\right)-\left(-\frac{15}{12}+\frac{6}{11}-\frac{48}{49}\right)\)
\(=\frac{-1}{4}+\frac{7}{33}-\frac{5}{3}+\frac{15}{12}-\frac{6}{11}+\frac{48}{49}\)
\(=\left(-\frac{1}{4}-\frac{5}{3}+\frac{15}{12}\right)+\left(\frac{7}{33}-\frac{6}{11}\right)+\frac{48}{49}\)
\(=\left(-\frac{23}{12}+\frac{15}{12}\right)+\left(\frac{7}{33}-\frac{18}{33}\right)+\frac{48}{49}\)
\(=-\frac{2}{3}+\frac{-1}{3}+\frac{48}{49}\)
\(=-1+\frac{48}{49}\)
\(=-\frac{49}{49}+\frac{48}{49}\)
\(=-\frac{1}{49}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
3,5,7,9,11,13,15,17,19,21,23,25,27,29,31,33,35,37,39,41 nhé
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:
\(\frac{22}{49}>\frac{3}{7}\)
Mà \(\frac{3}{7}>\frac{3}{8}\)
=>\(\frac{22}{49}>\frac{3}{8}\)
Cái này có Phân số trung gian là:\(\frac{21}{49}=\frac{3}{7}\)
ta có :
\(\hept{\begin{cases}7=7\\21=3\times7\\15=3\times5\end{cases}\Rightarrow BCNN\left(7,21,15\right)=3\times5\times7=105}\)
BCNN của ( 7;21;15) = 210