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\(\dfrac{7}{12}+\dfrac{17}{18}\)
\(=\dfrac{21}{36}+\dfrac{34}{36}\)
\(=\dfrac{21+34}{36}\)
\(=\dfrac{55}{36}\)
So sánh phân số bằng cách nhanh nhất: a)2/15 và 8/17 b)12/17 và 13/18 c)9/20 và 3/7 d)17/18 và 15/19
![](https://rs.olm.vn/images/avt/0.png?1311)
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2/3+4/5=10/15+12/15=22/15
4/6+8/12=8/12+8/12=16/12=4/3
99/17+100/17=199/17
67/12-7/6=67/12-14/12=53/12
65/18-8/6=65/18-24/18=41/18
\(\frac{2}{3}+\frac{4}{5}=\frac{22}{15}\)
\(\frac{4}{6}+\frac{8}{12}=\frac{4}{3}\)
\(\frac{99}{17}+\frac{100}{17}=\frac{199}{17}\)
\(\frac{67}{12}-\frac{7}{6}=\frac{53}{12}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+10+20
=20x10+10
=210
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b)\(\left(\dfrac{13}{21}\times\dfrac{7}{8}-\dfrac{12}{21}\times\dfrac{7}{8}\right)\times12\)
=\(\left(\dfrac{7}{8}\times\left(\dfrac{13}{21}-\dfrac{12}{21}\right)\right)\times12\)
=\(\left(\dfrac{7}{8}\times\dfrac{1}{21}\right)\times12\)
=\(\dfrac{7}{16}\)
c)\(\dfrac{7}{11}+\dfrac{3}{4}+\dfrac{4}{11}+\dfrac{8}{17}+\dfrac{1}{4}+\dfrac{18}{34}\)
= \(\left(\dfrac{7}{11}+\dfrac{4}{11}\right)+\left(\dfrac{3}{4}+\dfrac{1}{4}\right)+\dfrac{8}{17}+\dfrac{9}{17}\)
= 1 + 1 + \(\left(\dfrac{8}{17}+\dfrac{9}{17}\right)\)
= 2 + 1
= 3
![](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)
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20=210 nha bn
![](https://rs.olm.vn/images/avt/0.png?1311)
55/36
7/12 + 17/18
= 21/36 + 34/36
= 55/36