em lớp 6  nha

B= 1/2 + 1/6 + 1/12 +1/20 + 1/30 + 1/42 + 1/56 + 1/72

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

B=1-1/2+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

B=1+0-0-0-0-0-0-0-1/9

B=1-1/9

B=8/9

k em nha

phần A em chịu

còn câu dưới đó e

1/20+1/30+1/42+1/56+1/72

=1/4x5+1/5x6+1/6x7+1/7x8+1/8x9

=1/4-1/5+1/5-1/6+...+1/9

=1/4-1/9

= 5 / 36

= 1/4x5 + 1/5x6 + 1/6x7 + 1/7x8 + 1/8x9
=1/4 - 1/5 + 1/5 - 1/6 +... + 1/8 - 1/9
= 1/4 - 1/9 = 5/36

  1/30+1/42+1/56+1/72+....+1/210

=1/5.6+1/6.7+1/7.8+1/8.9+...+1/14.15

=1/5-1/6+1/6-1/7+1/7-1/8+....+1/14-1/15

=1/5-1/15

=2/15

=5/36

đúng thì k mình nhé 

làm ơn giúp mình mình đang bị âm

A = 1/20 +   1/30 + 1/42   +   1/56  + 1/72

A= 1/4x5 + 1/5x6 + 1/6x7 + 1/7x8 + 1/8x9

A= 1/4 - 1/5 + ......................+ 1/8 - 1/9

A= 1/4 - 1/9

A=     5/36

\(A=\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)

\(A=\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{7.8}+\frac{1}{8.9}\)

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

\(A=\frac{1}{4}-\frac{1}{9}\)

\(\Rightarrow A=\frac{5}{36}\)

Sửa đề: A=-1/20+(-1/30)+(-1/42)+(-1/56)+(-1/72)+(-1/90)

=-(1/20+1/30+...+1/90)

=-(1/4-1/5+1/5-1/6+...+1/9-1/10)

=-1/4+1/10

=-5/20+2/20=-3/20

\(\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}+\frac{1}{10.11}=\frac{1}{5}-\frac{1}{11}=\frac{6}{11}\)

1/5*6+1/6*7+1/8*9+1/9*10+1/10*11

1/5-1/6+1/6-1/7+....+1/10-1/11

1/5-1/10

1/10

1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110

= 1/5×6 + 1/6×7 + 1/7×8 + 1/8×9 + 1/9×10 + 1/10×11

= 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10 + 1/10 - 1/11

= 1/5 - 1/11

= 11/55 - 5/55

= 6/55

\(\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}+\frac{1}{110}\)

\(=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}+\frac{1}{10.11}\)

\(=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}+\frac{1}{10}-\frac{1}{11}\)

\(=\frac{1}{5}-\frac{1}{11}\)

\(=\frac{6}{55}\)

1/30 +1/42 +1/56+1/72+1/90+1/110

=1/ 5.6 +1/6.7 +1/7.8 +1/8.9 +1/ 9.10 +1/10.11

=1. (1/5 - 1/6 +1/6 -1/7+1/7 -1/8 +1/8 -1/9 +1/9 - 1/10 +1/10 - 1/11 )

=1.(1/5 -1/11)

=1.6/55 =6.55

k nha!!! Cái này mk học rồi