tính giúp mình bài này
B=2/15+2/35+2/63+...+2/575
K
Khách
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Những câu hỏi liên quan
20 tháng 4 2016
= 2/3×5 + 2/ 5×7 + 2/ 7×9 + 2/ 9×11 + ...... + 2/ 23×25+ 2 / 25×27
=1/3 - 1/5 + 1/5 -1/7 + 1/7 -1/9 +1/9 -1/11+.......+1/23- 1/25 + 1/25 -1/27
= 1/3 - ( 1/5+1/5 -1/7 +1/7 - 1/9 +1/9-1/11 +........+ 1/23 -1/25 + 1/25 )-1/27
=1/3 - 1/27= 9/27 - 1/27= 8/27
17 tháng 2 2015
A= 2( 1/15 + 1/35 + 1/63+ 1/99+1/143)
A= 2(1/3x5 +1/5x7 + 1/7x9 + 1/9x11 + 1/11x13)
A=2(1/3-1/5+1/5-1/+1/7-1/9+1/9-1/11+1/11-1/13)
A=2(1/3-1/13)
A=2x10/39
A=20/39
2 tháng 9 2017
A = 2/15 + 2/35 + 2/63 + 2/99 + 2/143
A = 2/3x5 + 2/5x7 + 2/7x9 + 2/9x11 + 2/11x13
A = 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + 1/11 - 1/13
A = 1/3 - 1/13
A = 12/13
19 tháng 3 2016
=[2/3+2/99]+[2/35+2/63]+2/15
=24/99+[4/45+2/15]
=24/99+2/9
=46/99
LC
19 tháng 3 2016
\(=\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+\frac{2}{7\times9}+\frac{2}{9\times11}\)
\(=\frac{1}{2}\times\left(\frac{2}{1}-\frac{2}{3}+\frac{2}{3}-\frac{2}{5}+\frac{2}{5}-\frac{2}{7}+\frac{2}{7}-\frac{2}{9}+\frac{2}{9}-\frac{2}{11}\right)\)
\(=\frac{1}{2}\times\frac{20}{11}=\frac{10}{11}\)
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 ! >_<
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 \(=\) \(\dfrac{2}{15}\) \(+\) \(\dfrac{2}{35}\) \(+\) \(\dfrac{2}{63}\) \(+\) . . . . . \(+\) \(\dfrac{2}{575}\)
\(=\) \(\dfrac{2}{3.5}\) \(+\) \(\dfrac{2}{5.7}\) \(+\) \(\dfrac{2}{7.9}\) \(+\) . . . . . \(+\) \(\dfrac{2}{23.25}\)
\(=\) \(\dfrac{1}{3}\) \(-\) \(\dfrac{1}{5}\) \(+\) \(\dfrac{1}{5}\) \(-\) \(\dfrac{1}{7}\) \(+\) \(\dfrac{1}{7}\) \(-\) \(\dfrac{1}{9}\) \(+\) . . . . . \(+\) \(\dfrac{1}{23}\) \(-\) \(\dfrac{1}{25}\)
\(=\) \(\dfrac{1}{3}\) \(-\) \(\dfrac{1}{25}\)
\(=\) \(\dfrac{22}{75}\)
NT
3
16 tháng 3 2017
\(-2\left(\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}+...+\frac{1}{9999}\right)\)
\(=-2\left(\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}+\frac{1}{99.101}\right)\)\(=-2\cdot\left(\frac{1}{3}-\frac{1}{101}\right)\)
=.....
mình quên đem máy tính nên k ghi đc đấp số
THÔNG CẢM
VN
3
29 tháng 4 2019
+ \(\frac{1}{n\times\left(n+2\right)}=\frac{\left(n+2\right)-n}{n\times\left(n+2\right)}\)
\(=\frac{n+2}{n\times\left(n+2\right)}-\frac{n}{n\times\left(n+2\right)}=\frac{1}{n}-\frac{1}{n+2}\)
+ \(\frac{2}{3}+\frac{14}{15}+\frac{34}{35}+\frac{62}{63}+\frac{98}{99}\)
\(=1-\frac{1}{3}+1-\frac{1}{15}+1-\frac{1}{35}+1-\frac{1}{63}+1-\frac{1}{99}\)
\(=5-\left(\frac{1}{3}+\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}\right)\)
\(=5-\frac{1}{2}\times\left(\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+\frac{2}{7\times9}+\frac{2}{9\times11}\right)\)
\(=5-\frac{1}{2}\times\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{11}\right)\)
\(=5-\frac{1}{2}\times\left(1-\frac{1}{11}\right)\)
\(=5-\frac{1}{2}+\frac{1}{22}=\frac{50}{11}\)
TT
3
S
29 tháng 4 2019
\(\left(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}\right).Y=4\)
\(\Rightarrow\frac{2}{9}.Y=4\)
\(\Rightarrow Y=4:\frac{2}{9}\)
\(\Rightarrow Y=18\)
Vậy số cần tìm là 18.
~Study well~
29 tháng 4 2019
\(\left(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}\right).y=4\)
\(\left(\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}\right).y=4\)
\(\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}\right).y=4\)
\(\left(\frac{1}{3}-\frac{1}{9}\right).y=4\)
\(\frac{2}{9}.y=4\)
\(y=18\)
Vậy \(y=18\)
NT
3
30 tháng 6 2019
\(\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+\frac{2}{63}=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}\)
\(=1-\frac{1}{9}=\frac{8}{9}\)
HL
30 tháng 6 2019
\(\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+\frac{2}{63}\)
\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}\)( chỗ này tối giản nha )
\(=1-\frac{1}{9}\)
\(=\frac{8}{9}\)
~ Hok tốt ~
\(B=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{575}\)
\(=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{23.25}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{23}-\frac{1}{25}\)
\(=\frac{1}{3}-\frac{1}{25}=\frac{22}{75}\)