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

giúp

=> (1/1.2 + 1/2.3 + 1/3.4 + ... + 1/8.9 + 1/9.10) : x = 9/20

=> (1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/8 - 1/9 + 1/9 - 1/10) : x = 9/20

=> (1 - 1/10) : x = 9/20

=> 9/10 : x = 9/20

X = 9/10 : 9/20 = 2

hhhhhhijijkn

(1/2+1/6+1/12+1/20+...+1/72) : X = 3/4

(1/1x2+1/2x3+1/3x4+1/4x5+...+1/7x8) : X = 3/4

(1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+...+1/7-1/8) : X = 3/4

(1/1-1/8) : X = 3/4

7/8 : X = 3/4

X = 7/8 : 3/4

X = 7/6

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

\(=\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}\)

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

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

\(=\frac{7}{18}\)

1/2+1/6+...+1/72=1/1x2+1/2x3+...+1/8x9=1/1-1/2+1/2-1/3+...+1/8-1/9=1-1/9=8/9

\(\text{1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90}\)

\(\text{= 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.1}\)

\(\text{= 1/1 - 1/2 + 1/2 - 1/3 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10}\)

\(=1/1-1/10-10/10-1/10-9/10\)

Vậy \(\text{ 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 = 9/10}\)

Sửa đề:

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

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

\(=\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{9}-\dfrac{1}{10}\)

\(=\dfrac{1}{2}-\dfrac{1}{10}\)

\(=\dfrac{3}{5}\)

A=1/2+1/6+1/12+1/20+...+1/72+1/90

A=1/1x2+1/2x3+1/3x4+1/4x5+.....+1/8x9+1/9x10

A=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+....+1/8-1/9+1/9-1/10

A=1-1/10

A=9/10

k cho mik nha

A=1/(1*2)+1/(2*3)+1/(3*4)+.....+1/(9*10)

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

=1/1-1/10=9/10

A=1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10

A=1/2-1/3+1/3-1/4+1/4-1/5+...+1/9-1/10

A=1/2-1/10

A=2/5