giúp mình đi mình tích lun đó

A=2/6+2/12+...+2/90

  =2/2x3+2/3x4+..+2/9x10

  =2(1/2-1/3+1/3-1/4+...+1/9-1/10)

  =2(1/2-1/10)=2x2/5=4/5.

:-((

1/2+1/6+1/12+...+1/90=1/(1.2)+1/(2.3)+1/(3.4)+...+1/(9.10)

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

                                =1-1/10

                                =9/10

9/10 nha bạn

\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+..+\frac{1}{90}=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\)

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

dấu "." là nhân nhé

\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\)

\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\)

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

\(A=1-\frac{1}{10}\)

\(A=\frac{9}{10}\)

1/2+1/6+1/12+1/20+...+1/90

=1/1.2+1/2.3+1/3.4+...+1/9.10

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

=1-1/10

=9/10

\(\Rightarrow\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}...+\frac{1}{9.10}\)

=\(\frac{9}{10}\)

1/2+5/6+11/12+19/20+...+89/90

                    Đ/s: 81/10.

Đặt A = 1/2 + 5/6 + 11/12 + 19/20 + ... + 89/90

A = ( 1 - 1/2 ) + ( 1 - 1/12 ) + ( 1 - 1/20 ) + ... + ( 1 - 1/90 )

A = ( 1 + 1 + 1 + ... + 1 + 1 ) - ( 1/2 + 1/6 + 1/20 + ... + 1/90

A = 9 - ( 1/1.2 + 1/2.3 + 1/3.4 + ... + 1/9.10 )

A = 9 - ( 1 - 1/10 )

A = 9 - 9/10

A = 81/10

bạn xem  lại đề bài vs ạ 91/2 hay là 1/2 ạ

bạn ơi bn xem lại gíup mk 91/2 hay 1/2 tớ mới giải đc

nếu mà 1/2 thì tớ giải

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

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