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A=5.(2.3+4.9.5+6.9.7+10.21.8)/7.(2.3+4.9.5+6.9.7+10.21.8)
A=5/7
![](https://rs.olm.vn/images/avt/0.png?1311)
A = \(\dfrac{2.3.5+4.9.35+6.9.35+10.21.40}{2.3.7+4.9.35+6.9.49+10.21.56}\)
A = \(\dfrac{2.3.5+\left(2.3.5\right).2.3.5+\left(2.3.5\right).3.3.7+\left(2.3.5\right).5.7.8}{2.3.7+\left(2.3.7\right).2.3.5+\left(2.3.7\right).3.3.7+\left(2.3.7\right).5.7.8}\)
A = \(\dfrac{2.3.5.\left(1+2.3.5+3.3.7+5.7.8\right)}{2.3.7\left(1+2.3.5+3.3.7+5.7.8\right)}\)
A = \(\dfrac{2.3.5}{2.3.7}\)
A = \(\dfrac{5}{7}\)
Ta có: \(A=\dfrac{2\cdot3\cdot5+4\cdot9\cdot35+6\cdot9\cdot35+10\cdot21\cdot40}{2\cdot3\cdot7+4\cdot9\cdot35+6\cdot9\cdot49+10\cdot21\cdot56}\)
\(=\dfrac{2\cdot3\cdot5+2\cdot3\cdot5\cdot6\cdot7+2\cdot3\cdot5\cdot3\cdot3\cdot7+2\cdot3\cdot5\cdot5\cdot7\cdot8}{2\cdot3\cdot7+2\cdot3\cdot7\cdot2\cdot3\cdot5+2\cdot3\cdot7\cdot3\cdot3\cdot7+2\cdot3\cdot7\cdot5\cdot7\cdot8}\)
\(=\dfrac{2\cdot3\cdot5\cdot\left(1+2\cdot3\cdot7+3\cdot3\cdot7+5\cdot7\cdot8\right)}{2\cdot3\cdot7\cdot\left(1+2\cdot3\cdot5+3\cdot3\cdot7+5\cdot7\cdot8\right)}\)
\(=\dfrac{5}{7}\cdot\dfrac{386}{374}=\dfrac{965}{1309}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có: 5.7.9.11 ⋮7 và 2.3.7 ⋮7 nên (5.7.9.11 – 2.3.7) ⋮7
Vì 5.9.11 > 2.3 ⇒ 5.9.11 – 2.3 > 1 nên (5.7.9.11 – 2.3.7) > 7
Vậy 5.7.9.11 – 2.3.7 là hợp số
![](https://rs.olm.vn/images/avt/0.png?1311)
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đúng rồi đó bạn ! quá giỏi ! mình còn không làm được !
\(14\cdot x-2\cdot7^2=2\cdot3\cdot7\)
\(\Rightarrow14x=2\cdot3\cdot7+2\cdot7^2\)
\(\Rightarrow14x=2\cdot7\left(3+7\right)\)
\(\Rightarrow14x=14\cdot10\)
\(\Rightarrow14x=140\)
\(\Rightarrow x=\dfrac{140}{14}=10\)
#Toru