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NP
1
20 tháng 6 2020
\(B=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{575}\)
\(=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{23.25}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{23}-\frac{1}{25}\)
\(=\frac{1}{3}-\frac{1}{25}=\frac{22}{75}\)
KL
3
25 tháng 10 2021
a) \(\left(x-15\right)-75=0\\ \Leftrightarrow x-15=75\\ \Leftrightarrow x=90\)
b) \(575-\left(6x+70\right)=445\\ \Leftrightarrow6x+70=130\\ \Leftrightarrow6x=60\\ \Leftrightarrow x=10\)
c) \(x-105:21=15\\ \Leftrightarrow x-5=15\\ \Leftrightarrow x=20\)
d) \(\left(x-105\right):21=15\\ \Leftrightarrow x-105=315\\ \Leftrightarrow x=420\)
MH
25 tháng 10 2021
a) ( x - 15 ) - 75 = 0
x -15 = 75 + 0
x - 15 = 75
x= 75 + 15
x = 90
b) 575- (6x +70) =445
6x + 70 = 575 - 545
6x + 70 = 130
6x = 130 - 70
6x = 60
x = 60 : 6
x = 10
c) x –105 : 21 =15
x –105 = 15 x 21
x -105 = 315
x = 315 + 105
x = 420
d) (x - 105) :21 = 15
x - 105 = 21
x = 105 + 21
x = 126
13 tháng 8 2015
2/35 + 4/77 + 2/143 + 4/221 + 2/323 + 4/437 + 2/575
= 2/5.7 + 4/7.11 + 2/11.13 + 4/13.17 + 2/17.19 + 4/19.23 + 2/23.25
= 1/5 - 1/7 + 1/7 - 1/11 + 1/11 - 1/13 + 1/13 - 1/17 + 1/17 - 1/19 + 1/19 - 1/23 + 1/23 - 1/15
= 1/5 - 1/25
= 4/25
li ke cho mk nha
GC
2
KN
28 tháng 4 2019
\(A=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{575}\)
\(\Rightarrow A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{23.25}\)
\(\Rightarrow A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{5}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{23}-\frac{1}{25}\)
\(\Rightarrow A=\frac{1}{3}-\frac{1}{25}\)
\(\Rightarrow A=\frac{25}{75}-\frac{3}{75}\)
\(\Rightarrow A=\frac{22}{75}\)
28 tháng 4 2019
\(A=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{575}\)
\(=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{23.25}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{23}-\frac{1}{25}\)
\(=\frac{1}{3}-\frac{1}{25}\)
\(=\frac{22}{75}\)
Study well ! >_<
MT
14 tháng 7 2015
\(J=\frac{2}{35}+\frac{4}{77}+\frac{2}{143}+\frac{4}{221}+\frac{2}{323}+\frac{4}{437}+\frac{2}{575}\)
\(=\frac{2}{5\times7}+\frac{4}{7\times11}+\frac{2}{11\times13}+\frac{4}{13\times17}+\frac{2}{17\times19}+\frac{4}{19\times23}+\frac{2}{23\times25}\)
\(=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{17}+\frac{1}{17}-\frac{1}{19}+\frac{1}{19}-\frac{1}{23}+\frac{1}{23}-\frac{1}{25}\)
\(=\frac{1}{5}-\frac{1}{25}=\frac{5}{25}-\frac{1}{25}=\frac{4}{25}\)
H
1
13 tháng 8 2016
\(\frac{2}{35}+\frac{4}{77}+\frac{2}{143}+\frac{4}{221}+\frac{2}{323}+\frac{4}{437}+\frac{2}{575}\)
\(=\frac{2}{5.7}+\frac{4}{7.11}+\frac{2}{11.13}+\frac{4}{13.17}+\frac{2}{17.19}+\frac{4}{19.23}+\frac{2}{23.25}\)
\(=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{17}+\frac{1}{17}-\frac{1}{19}+\frac{1}{19}-\frac{1}{23}+\frac{1}{23}-\frac{1}{25}\)
\(=\frac{1}{5}-\frac{1}{25}\)
\(=\frac{5}{25}-\frac{1}{25}=\frac{4}{25}\)
PT
1
MT
21 tháng 6 2015
a,(x-15)-75=0
x-15 =75
x =75+15
x =90
b,72-(6.x+5)=55
6.x+5=72-55
6.x+5=17
6.x=17-5
6.x=12
x=12:6
x=2
c,575-(6.x+70)=445
6.x+70=575-445
6.x+70=130
6.x=130-70
6.x=60
x=60:6
x=10
d,(x-34).15=0
x-34=0
x=34
e,15+(18-x)=26
18-x=26-15
18-x=11
x=18-11
x=7
g,(x-5):16=2
x-5=2.16
x-5=32
x=32+5
x=37
ND
2
25 tháng 4 2018
\(A=\dfrac{2}{15}+\dfrac{2}{35}+...+\dfrac{2}{575}\\ =\dfrac{2}{3\cdot5}+\dfrac{2}{5\cdot7}+...+\dfrac{2}{23\cdot25}\\ =\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{23}-\dfrac{1}{25}\\ =\dfrac{1}{3}-\dfrac{1}{25}\\ =\dfrac{25}{75}-\dfrac{3}{75}\\ =\dfrac{22}{75}\)
MY
26 tháng 4 2018
A \(=\) \(\dfrac{2}{15}\) \(+\) \(\dfrac{2}{35}\) \(+\) \(\dfrac{2}{63}\) \(+\) . . . . . \(+\) \(\dfrac{2}{575}\)
\(=\) \(\dfrac{2}{3.5}\) \(+\) \(\dfrac{2}{5.7}\) \(+\) \(\dfrac{2}{7.9}\) \(+\) . . . . . \(+\) \(\dfrac{2}{23.25}\)
\(=\) \(\dfrac{1}{3}\) \(-\) \(\dfrac{1}{5}\) \(+\) \(\dfrac{1}{5}\) \(-\) \(\dfrac{1}{7}\) \(+\) \(\dfrac{1}{7}\) \(-\) \(\dfrac{1}{9}\) \(+\) . . . . . \(+\) \(\dfrac{1}{23}\) \(-\) \(\dfrac{1}{25}\)
\(=\) \(\dfrac{1}{3}\) \(-\) \(\dfrac{1}{25}\)
\(=\) \(\dfrac{22}{75}\)
H
2
NT
18 tháng 7 2018
2/35 + 4/77 + 2/143 + 4/221 + 2/323 + 4/437 + 2/575
= 2/5.7 + 4/7.11 + 2/11.13 + 4/13.17 + 2/17.19 + 4/19.23 + 2/23.25
= 1/5 - 1/7 + 1/7 - 1/11 + 1/11 - 1/13 + 1/13 - 1/17 + 1/17 - 1/19 + 1/19 - 1/23 + 1/23 - 1/15
= 1/5 - 1/25
= 4/25
18 tháng 7 2018
\(\frac{2}{35}\)+ \(\frac{4}{77}\)+ \(\frac{2}{143}\)+ \(\frac{4}{221}\)+ \(\frac{2}{323}\)+ \(\frac{4}{437}\)+ \(\frac{2}{575}\).
= \(\frac{2}{5.7}\)+ \(\frac{4}{7.11}\)+ \(\frac{2}{11.13}\)+ \(\frac{4}{13.17}\)+ \(\frac{4}{17.21}\)+ \(\frac{2}{21.23}\).
= \(\frac{1}{5}\)- \(\frac{1}{7}\)+ \(\frac{1}{7}\)- \(\frac{1}{11}\)+ \(\frac{1}{11}\)- \(\frac{1}{13}\)+ \(\frac{1}{13}\)- \(\frac{1}{17}\)+ \(\frac{1}{17}\)- \(\frac{1}{21}\)+ \(\frac{1}{21}\)- \(\frac{1}{23}\).
= \(\frac{1}{5}\)- \(\frac{1}{23}\).
= \(\frac{23}{115}\)- \(\frac{5}{115}\).
= \(\frac{18}{115}\).