bài này dễ mà

\(A=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}\) 

\(A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{19.21}\)

\(A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}\)

\(A=\frac{1}{3}-\frac{1}{21}=\frac{2}{7}\)

2/15 + 2/35 + 2/63 + ... + 2/399

= 2/3×5 + 2/5×7 + 2/7×9 + ... + 2/19×21

= 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + ... + 1/19 - 1/21

= 1/3 - 1/21

= 7/21 - 1/21

= 6/21 = 2/7

\(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+..+\frac{2}{399}\)

\(=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{19.21}\)

\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\)

\(=\frac{1}{3}-\frac{1}{21}=\frac{7}{21}-\frac{1}{21}=\frac{6}{21}=\frac{2}{7}\)

Ủng hộ mk nha !!! ^_^

2/7 bạn ak

\(B=\frac{2}{3.5}+\frac{2}{5.7}+......+\frac{2}{29.31}\)

\(B=2.\left(\frac{1}{3.5}+\frac{1}{5.7}+....+\frac{1}{29.31}\right)\)

\(B=2.\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+.....+\frac{1}{29}-\frac{1}{31}\right)\)

\(B=2.\left(\frac{1}{3}-\frac{1}{31}\right)\)

\(B=2.\frac{28}{93}\)

\(B=\frac{56}{93}\)

k cho mình nha chúc bạn học tốt

sai đề rồi bạn ơi!bạn hãy kiểm tra lại bài đi nha

`A =2/15 +2/35 +2/63 +... +2/339`

`= 2/(3.5) +2/(5.7) + 2/(7.9) + ...+2/(19.21)`

`= 1/3 -1/5 +1/5 -1/7 +1/7 -1/9 +... 1/19 -1/21`

`= 1/3 -1/21 = 7/21 -1/21`

`=6/21 = 2/7`

$A=2/15+2/35+2/63+...+2/339$

$=2/(3.5)+2/(5.7)+2/(7.9)+...+2/(19.21)$

$=1/3-1/5+1/5-1/7+1/7-1/9+...1/19-1/21$

$=1/3-1/21=7/21-1/21$

$=6/21=2/7$

sai rồi,2/7 mới đúng,bài này không cần nhân 2

ta có 

A=\(\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{19.21}=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}=\frac{1}{3}-\frac{1}{21}=\frac{2}{7}\)

\(=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+....+\frac{2}{19.21}\)

\(=2\left(\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{19.21}\right)\)

\(=2\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\right)\)

\(=2\left(\frac{1}{3}-\frac{1}{21}\right)\)

=\(\frac{4}{7}\)

\(A=\)\(\frac{2}{3.5}+\frac{2}{5.7}+.....+\frac{2}{19.21}\)\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+....+\frac{1}{19}-\frac{1}{21}=\frac{1}{3}-\frac{1}{21}=\frac{2}{7}\)

\(y=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{19.21}\)

\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\)

\(=\frac{1}{3}-\frac{1}{21}=\frac{2}{7}\)

mình thắc mắc quy luật của phép tính trên là gì : 15 -> 35 -> 63 ... -> 399 ?

Quy luật la cac so o duoi mau cach nhau 2 don vi ( cach nhau mot so bang tu):

2/15=2/3.5 ( 3 va 5 cach nhau 2don vi)

2/35=2/5.7 ( 5 va 7 cach nhau 2 don vi)

.........

(Cai nay minh gui cho ban Duy Thanh nhe!!!!!!!!!!!!!!!!!!!)

     D =3-3²-....-3¹⁰⁰

(=)D=3-(3²+3³+....+3¹⁰⁰)

(=)3D=3.3-(3³+....+3¹⁰¹)

(=)2D=6-(3¹⁰¹-3²)

(=)D=6-(3¹⁰¹-3²) :2

     B=-2/15-2/35-....-2/399

(=)B=-2/15-(2/35-...-2/399)

(=)B=-2/15-(2/5.7-...-2/19.21)

(=)B=-2/15-(1/5-1/21)

(=)B=-2/15-16/105

(=)B=-2/7