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19 tháng 1 2018

ta có

(1/3+1/6+1/36) +(1/10+1/15+1/45)+(1/21+1/28)

=(\(\frac{12+6+1}{36}\)+\(\frac{9+6+2}{90}\)+\(\frac{4+3}{84}\)

19/36+17/90+1/12

=(19/36+1/12)+17/90

=7/12+17/90

=105/180+34/180

=139/180

19 tháng 1 2018

1/3 +1/6+1/10+1/15+1/21+1/28+1/36+1/45

=1/1x3+1/3x2+1/2x5+1/3x5+1/3x7+1/7x4+1/4x9+1/9x5

=1/1-1/3+1/3-1/2....+1/9-1/5

=1/1

24 tháng 4 2016

P=2(1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90)

P=2((1/2*3)+(1/3*4)+(1/4*5)+(1/5*6)+(1/6*7)+(1/7*8)+(1/8*9)+(1/9*10)

P=2(1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10)

P=2(1/2-1/10)

P=2*2/5

P=4/5

H NHA.

18 tháng 6 2016

Ta có:

\(A=\frac{1}{3}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{195}\)

\(\Rightarrow2A=\frac{2}{3}+\frac{2}{15}+\frac{2}{21}+...+\frac{2}{195}\)

\(=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{13.15}\)

\(=\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{13}-\frac{1}{15}\)

\(=\frac{1}{1}-\frac{1}{15}\)

\(=\frac{14}{15}\)

\(\Rightarrow2A=\frac{14}{15}\Rightarrow A=\frac{14}{15}\div2=\frac{7}{15}\)

Vậy A = 7/15

18 tháng 6 2016

hình như đề sai ở ps cuối

5 tháng 9 2021

\(P=\dfrac{1}{3}+\dfrac{1}{6}+\dfrac{1}{10}+\dfrac{1}{15}+\dfrac{1}{21}+\dfrac{1}{28}+\dfrac{1}{36}+\dfrac{1}{45}\)

\(=2\left(\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}\right)\)

\(=2\left(\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}\right)\)

\(=2\left(\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{9}-\dfrac{1}{10}\right)\)

\(=2\left(\dfrac{1}{2}-\dfrac{1}{10}\right)=2.\dfrac{2}{5}=\dfrac{4}{5}\)

5 tháng 9 2021

bn cộng hết lại vs nhau là ra kq!

11 tháng 4 2016

2) A = \(\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+\frac{1}{36}\)

=> \(\frac{1}{2}\).A = \(\frac{1}{2}\).\(\left(\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+\frac{1}{36}\right)\)

=> \(\frac{1}{2}\).A = \(\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)

=> \(\frac{1}{2}\).A = \(\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}\)

=> \(\frac{1}{2}\).A = \(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)

=> \(\frac{1}{2}\).A = \(\frac{1}{3}-\frac{1}{9}\)

=> \(\frac{1}{2}\).A = \(\frac{2}{9}\)

=> A = \(\frac{2}{9}:\frac{1}{2}\)

=> A = \(\frac{4}{9}\)

11 tháng 4 2016

chang hieu cau hoi gi

A=2(1/2+1/6+...+1/90)

=2(1-1/2+1/2-1/3+...+1/9-1/10)

=2*9/10=9/5<2

17 tháng 4 2016

\(\frac{1}{2}\) E= \(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)

\(\frac{1}{2}\) E = \(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{8.9}\)

\(\frac{1}{2}E\) = \(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{8}-\frac{1}{9}\)

\(\frac{1}{2}E\) = \(\frac{1}{2}-\frac{1}{9}\)

\(\frac{1}{2}E\) =\(\frac{7}{18}\)

=> E = \(\frac{7}{9}\)

17 tháng 4 2016

E=\(\frac{1}{3}+\frac{1}{6}+....+\frac{1}{28}+\frac{1}{36}\)

\(\frac{1}{2}E=\frac{1}{6}+\frac{1}{12}+...+\frac{1}{56}+\frac{1}{72}\)

\(\frac{1}{2}E=\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{7.8}+\frac{1}{8.9}\)

\(\frac{1}{2}E=\frac{3-2}{2.3}+\frac{4-3}{3.4}+...\frac{8-7}{7.8}+\frac{9-8}{8.9}\)

\(\frac{1}{2}E=\frac{3}{2.3}-\frac{2}{2.3}+\frac{4}{3.4}-\frac{3}{3.4}+...+\frac{8}{7.8}-\frac{7}{7.8}+\frac{9}{8.9}-\frac{8}{8.9}\)

\(\frac{1}{2}E=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)

\(\frac{1}{2}E=\frac{1}{2}-\frac{1}{9}=\frac{7}{18}\)

E=\(\frac{7}{18}:\frac{1}{2}=\frac{7}{9}\)

15 tháng 4 2018

7/15 nhé bạn!

15 tháng 4 2018

đặt A=1/6+1/10+1/15+1/21+1/28+1/36+1/45

A*2=(1/6*+1/10+1/15+1/21+1/28+1/36+1/45)*2

A*2=1/12+1/20+1/30+1/42+1/56+1/72+1/90

A*2=1/3*4+1/4*5+1/5*6+1/6*7+1/7*8+1/8*9+1/9*10

A*2=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-/8+1/8-1/9+1/9-1/10

A*2=1/3-1/10

A*2=7/30

A=7/30 / 2

A=7/15

15 tháng 4 2018

\(\frac{4}{5}\)

15 tháng 4 2018

đặt A=1/6+1/10+1/15+1/21+1/28+1/36+1/45

6A=1+3/5+2/5+2/7+3/14+1/6+2/15

6A=1+1+7/14+1/6+2/15

6A=14/5

A=14/5:6=7/15

20 tháng 5 2020

=0,(428571)

HOK TỐT

25 tháng 5 2020

=0,(428571)

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