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`1/2+2/4+3/6+4/8+5/10+6/12`
`=1/2+1/2+1/2+1/2+1/2+1/2`
`=1/2*6=3`
`1/3+1/4+1/5+8/10+20/15+20/30`
`=(1/3+1/4)+(1/5+4/5)+(4/3+2/3)`
`=7/12+1+2`
`=7/12+3=43/12`
\(\dfrac{1}{2}+\dfrac{2}{4}+\dfrac{3}{6}+\dfrac{4}{8}+\dfrac{5}{10}+\dfrac{6}{12}\)
\(=\dfrac{1}{2}+\dfrac{1}{2}+\dfrac{1}{2}+\dfrac{1}{2}+\dfrac{1}{2}+\dfrac{1}{2}\)
\(=\dfrac{1}{2}\times6=3\)
\(------\)
\(\dfrac{1}{3}+\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{8}{10}+\dfrac{20}{15}+\dfrac{20}{30}\)
\(=\dfrac{1}{3}+\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{4}{5}+\dfrac{4}{3}+\dfrac{2}{3}\)
\(=\left(\dfrac{1}{3}+\dfrac{4}{3}+\dfrac{2}{3}\right)+\left(\dfrac{1}{5}+\dfrac{4}{5}\right)+\dfrac{1}{4}\)
\(=\dfrac{7}{3}+1+\dfrac{1}{4}\)
\(=\dfrac{28}{12}+\dfrac{12}{12}+\dfrac{3}{12}\)
\(=\dfrac{43}{12}\)
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\(\dfrac{3}{1\cdot3\cdot5}+\dfrac{3}{3\cdot5\cdot7}+\dfrac{3}{5\cdot7\cdot9}+\cdot\cdot\cdot+\dfrac{3}{37\cdot39\cdot41}\)
\(=\dfrac{3}{4}\left(\dfrac{4}{1\cdot3\cdot5}+\dfrac{4}{3\cdot5\cdot7}+\dfrac{4}{5\cdot7\cdot9}+\cdot\cdot\cdot+\dfrac{4}{37\cdot39\cdot41}\right)\)
\(=\dfrac{3}{4}\left(\dfrac{1}{1\cdot3}-\dfrac{1}{3\cdot5}+\dfrac{1}{3\cdot5}-\dfrac{1}{5\cdot7}+\dfrac{1}{5\cdot7}-\dfrac{1}{7\cdot9}+\cdot\cdot\cdot+\dfrac{1}{37\cdot39}-\dfrac{1}{39\cdot41}\right)\)
\(=\dfrac{3}{4}\left(\dfrac{1}{3}-\dfrac{1}{39\cdot41}\right)\)
\(=\dfrac{3}{4}\cdot\dfrac{532}{1599}=\dfrac{133}{533}\)
#Ayumu
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12/18 + 12/42 = 2/3 + 2/7 = 14/21 + 6/21 = 20/21
1/2 + 2/4 + 3/6 + 4/8 + 5/10 + 6/12
= 1/2 + 1/2 + 1/2 + 1/2 + 1/2 + 1/2
= 1/2 x 6
=6/2
=3
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1.
a, Ta có 28=14⇒14+56=1312(Bn tự quy đồng lên nhé !)
b,Ta có1216=34⇒18+34=78(Bn tự quy đồng lên nhé !)
c,Ta có636=16⇒14+16=512(Bn tự quy đồng lên nhé !)
d,Ta có1218=23⇒23+14=1112(Bn tự quy đồng lên nhé !)
7.Phần này mk chỉ cho đáp án thuia.67 ;
b,420=15 ;
c,615 ;
d,129
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`3/7 xx 1/9 + 3/7 xx 8/9`
`= 3/7 xx (1/9 +8/9)`
`= 3/7 xx 9/9`
`= 3/7 xx 1`
`=3/7`
`3/7xx1/9+3/7xx8/9`
`=3/7xx(1/9+8/9)`
`=3/7xx9/9`
`=3/7xx1`
`=3/7`
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Đề bài thiếu số hạng 3/20.
\(A=\dfrac{3}{2}+\dfrac{3}{6}+\dfrac{3}{12}+\dfrac{3}{20}+\dfrac{3}{30}+\dfrac{3}{42}\)
\(\dfrac{A}{3}=\dfrac{1}{1x2}+\dfrac{1}{2x3}+\dfrac{1}{3x4}+\dfrac{1}{4x5}+\dfrac{1}{5x6}+\dfrac{1}{6x7}=\)
\(=\dfrac{2-1}{1x2}+\dfrac{3-2}{2x3}+\dfrac{4-3}{3x4}+...+\dfrac{7-6}{6x7}=\)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{6}-\dfrac{1}{7}=\)
\(=1-\dfrac{1}{7}=\dfrac{6}{7}\Rightarrow A=3x\dfrac{6}{7}=\dfrac{18}{7}\)
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a) (15 +35) : 5 = ?
Cách 1: (15 +35) : 5
= 50 : 5 = 10
Cách 2: (15 +35) : 5
= 15 : 5 + 35 : 5
= 3 + 7 = 10
+) (80 +4) : 4 = ?
Cách 1:(80 +4) : 4
= 84 : 4 = 21
Cách 2:(80 +4) : 4
= 80 : 4 + 4 :4 = 20 +1 = 21
b) 18 : 6 + 24 : 6 = ?
Cách 1: 18 : 6 + 24 : 6
= 3 + 4 = 7
Cách 2: (18 + 24) : 6
= 42 : 6 = 7
+) 60 : 3 + 9 : 3 =?
Cách 1: 60 : 3 + 9 : 3
= 20 + 3 = 23
Cách 2: 60 : 3 + 9 : 3
=(60 +9) : 3 = 23
a) (15 +35) : 5 = ?
Cách 1: (15 +35) : 5
= 50 : 5 = 10
Cách 2: (15 +35) : 5
= 15 : 5 + 35 : 5
= 3 + 7 = 10
+) (80 +4) : 4 = ?
Cách 1:(80 +4) : 4
= 84 : 4 = 21
Cách 2:(80 +4) : 4
= 80 : 4 + 4 :4 = 20 +1 = 21
b) 18 : 6 + 24 : 6 = ?
Cách 1: 18 : 6 + 24 : 6
= 3 + 4 = 7
Cách 2: (18 + 24) : 6
= 42 : 6 = 7
+) 60 : 3 + 9 : 3 =?
Cách 1: 60 : 3 + 9 : 3
= 20 + 3 = 23
Cách 2: 60 : 3 + 9 : 3
=(60 +9) : 3 = 23
HT
3/2+3/6+3/12+3/20+3/30
=3/2+1/2+1/4+3/20+1/10
=30/20+10/20+5/20+3/20+2/20
=30+10+5+3+2/20
=50/20=5/2
\(\frac{3}{2}\)+\(\frac{3}{6}\)+\(\frac{3}{12}\)+ \(\frac{3}{20}\)+\(\frac{3}{30}\)
= \(\frac{3}{2}\) + \(\frac{1}{2}\)+ \(\frac{1}{4}\)+ \(\frac{3}{20}\)+ \(\frac{1}{10}\)
= \(\frac{30}{20}\)+ \(\frac{10}{20}\)+ \(\frac{5}{20}\)+\(\frac{3}{20}\)+\(\frac{2}{20}\)
= \(\frac{50}{20}\)= \(\frac{5}{2}\)