help me,sáng mai thầy kiểm tra bài của mình

Ta có : A = 2/ 4.7 + 2/ 7.10 + ... + 2/ 73.76 .

⇒ 3/2 A = 3/2 . ( 2/ 4.7 + 2/ 7.10 + ... + 2/ 73.76 ) .

⇒ 3/2 A = 3/ 4.7 + 3/ 7.10 + ... + 3/ 73.76 .

= 1/4 - 1/7 + 1/7 - 1/10 + ... + 1/ 73 - 1/76 .

= 1/4 - 1/76 .

= 19/76 - 1/76 .

= 9/38 .

Do đó : A = 9/38 : 3/2 .

= 9/38 . 2/3 .

= 3/19 .

Vậy A = 3/19 .

cảm ơn mọi người nhiều lắm

các bn trả lời nhanh nhé

đến 9:10 nhé

\(A=\frac{2}{4.7}+\frac{2}{7.10}+\frac{2}{10.13}+.....+\frac{2}{73.76}\)

\(=\frac{2}{3}\left(\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+....+\frac{3}{73.76}\right)\)

\(=\frac{2}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+....+\frac{1}{73}-\frac{1}{76}\right)\)

\(=\frac{2}{3}\left(\frac{1}{4}-\frac{1}{76}\right)\)

\(=\frac{2}{3}.\frac{9}{38}=\frac{3}{19}\)

A=\(\frac{3}{3}\cdot\left(\frac{2}{4\cdot7}+\frac{2}{7\cdot10}+\frac{2}{10\cdot13}+...+\frac{2}{73\cdot76}\right)\)

A=\(\frac{1}{3}\cdot3\cdot\left(\frac{2}{4\cdot7}+\frac{2}{7\cdot10}+\frac{2}{10\cdot13}+...+\frac{2}{73\cdot76}\right)\)

A=\(\frac{1}{3}\cdot\left(\frac{3}{4\cdot7}+\frac{3}{7\cdot10}+\frac{3}{10\cdot17}+...+\frac{3}{73\cdot76}\right)\)

A=\(\frac{1}{3}\cdot\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}-\frac{1}{13}+...+\frac{1}{73}-\frac{1}{76}\right)\)

A=\(\frac{1}{3}\cdot\left(\frac{1}{4}-\frac{1}{76}\right)\)

A=\(\frac{1}{3}\cdot\left(\frac{19}{76}-\frac{1}{76}\right)\)

A=\(\frac{1}{3}\cdot\frac{18}{76}\)

A=\(\frac{1\cdot6}{1\cdot76}\)

A=\(\frac{6}{76}\)

chúc bạn một ngày tốt lành

a) \(A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{37.39}\)

\(A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{37}-\frac{1}{39}\)

\(A=\frac{1}{3}-\frac{1}{39}\)

\(A=\frac{13}{39}-\frac{1}{39}=\frac{12}{39}\)

b) \(B=\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+...+\frac{3}{73.76}\)

\(B=\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{73}-\frac{1}{76}\)

\(B=\frac{1}{4}-\frac{1}{76}\)

\(B=\frac{19}{76}-\frac{1}{76}=\frac{18}{76}=\frac{9}{38}\)

a, \(A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{37.39}\)

\(A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{37}-\frac{1}{39}\)

\(A=\frac{1}{3}-\frac{1}{39}\)

\(A=\frac{4}{13}\)

b, \(B=\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+...+\frac{3}{73.76}\)

\(B=\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{73}-\frac{1}{76}\)

\(B=\frac{1}{4}-\frac{1}{76}\)

\(B=\frac{9}{38}\)

http://olm.vn/hoi-dap/question/772291.html

sau 3 phút có kết quả tùy bạn

A=\(\frac{37}{78}\)

B=\(\frac{9}{38}\)

A = 1 /1.2 + 1/ 2.3 + 1 /3.4 + . . . + 1/ 49.50 + 1/ 50.51

 A = 2 − 1/ 1.2 + 3 − 2 /2.3 + 4 − 3 /3.4 + . . . + 50 − 49 /49.50 + 51 − 50/ 50.51

A = 1 − 1/ 2 + 1/ 2 − 1 /3 + 1 /3 − 1/ 4 + . . . + 1 /50 − 1 /51

A=1-1/51

A=50/51

Cảm ơn bn

B=6/4.7+6/7.10+6/10.13+...+6/73.76

=2.(3/4.7 +3/7.10 +3/10.13 +...+3/73.76 )

=2.(1/4 −1/7 +1/7 −1/10 +1/10 −1/13 +...+1/73 −1/76 )

=2.(1/4-1/76)=2.9/38=9/19

A=\(\frac{1}{4.7}+\frac{1}{7.10}+\frac{1}{10.13}+...+\frac{1}{73.76}\)

3A=3(\(\frac{1}{4.7}+\frac{1}{7.10}+\frac{1}{10.13}+...+\frac{1}{73.76}\))

3A=\(\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+...+\frac{3}{73.76}\)

3A=\(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{73}+\frac{1}{76}\)

3A=\(\frac{1}{4}-\frac{1}{76}\)

3A=\(\frac{9}{38}\)

A=\(\frac{9}{38}\):3

A=\(\frac{3}{38}\)

đặt A=1/4.7+1/7.10+...+1/73.76

3A=1/4-1/7+1/7-1/10+...+1/ 73 -1/ 76
 

\(A=\frac{1}{4.7}+\frac{1}{7.10}+\frac{1}{10.13}+...+\frac{1}{73.76}\)

\(3A=\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+...+\frac{3}{73.76}\)

\(3A=\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{73}-\frac{1}{76}\)

\(3A=\frac{1}{4}-\frac{1}{76}\)

\(3A=\frac{9}{38}\)

\(A=\frac{3}{38}\)

a 

9/38 

chuc ban hoc tot

A=6/4.7+6/7.10+6/10.13+...+6/73.76

\(=2.\left(\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+...+\frac{3}{73.76}\right)\)

\(=2.\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{73}-\frac{1}{76}\right)\)

\(=2.\left(\frac{1}{4}-\frac{1}{76}\right)=2.\frac{9}{38}=\frac{9}{19}\) 

A=6/4.7+6/7.10 +...+6/73.76=6/4-6/7+6/7-6/10+...+6/73-6/76=6/4-6/76=27/19

\(A=\frac{6}{4.7}+\frac{6}{7.10}+\frac{6}{10.13}+...+\frac{6}{73.76}\)

\(\frac{1}{2}A=\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+...+\frac{3}{73.76}\)

\(\frac{1}{2}A=\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{73}-\frac{1}{76}\)

\(\frac{1}{2}A=\frac{1}{4}-\frac{1}{76}=\frac{9}{38}\)

\(\Rightarrow A=\frac{9}{38}:\frac{1}{2}=\frac{9}{19}\)