Question

Tính

A = \(\frac{3}{2}\) + \(\frac{3}{6}\) + \(\frac{3}{12}\) + \(\frac{3}{20}\) +....+ \(\frac{3}{90}\)

Giúp mình nhé!

Answer 1

Chào bạn, bạn theo dõi lời giải của mình nhé! 

Ta có : 

\(A=\frac{3}{2}+\frac{3}{6}+\frac{3}{12}+\frac{3}{20}+...+\frac{3}{90}\)

\(=>A=3.\frac{1}{2}+3.\frac{1}{6}+3.\frac{1}{12}+3.\frac{1}{20}+...+3.\frac{1}{90}\)

\(=>A=3.\left(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\right)\)

\(=>A=3.\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\right)\)

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

\(=>A=3.\left(1-\frac{1}{10}\right)=3.\frac{9}{10}=\frac{27}{10}\)

Có gì không hiểu bạn nhắn tin cho mình nhé! Chúc bạn học tốt!

Answer 2

Bạn tách mẫu thành

1.2 ; 2.3 ; 3.4;... 9.10

Tới đây biết làm chưa, chưa biết làm thì bảo mk mk giải cụ thể cho

Answer 3

= 3/1.2 + 3/2.3 + 3/3.4 + 3/4.5 + ... + 3/9.10

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

Answer 4

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

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

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

=3*(1-1/10)

=3*9/10

=27/10

Answer 5

giải

A=3/2+3/6+3/12+3/20+...+3/90

A=3/1.2+3/2.3+3/3.4+3/4.5+...+3/9.10

3A=1+

Answer 6

A = $\frac{3}{2}$‍32  + $\frac{3}{6}$36  + $\frac{3}{12}$312  + $\frac{3}{20}$320  +....+ $\frac{3}{90}$

Answer 7

\(A=\frac{3}{1.2}+\frac{3}{2.3}+\frac{3}{3.4}+\frac{3}{4.5}+...+\frac{3}{9.10}\)

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

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

\(A=\frac{10}{10}-\frac{1}{10}=\frac{9}{10}\)

Answer 8

1/3 A = 1/2 + 1/6 + 1/12 + ... + 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

A  = 9/10 : 1/3 = 9/10 * 3 = 3/10

Answer 9

A       = 3/2 + 3/6 + 3/12 + 3/20 + ... + 3/90

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

A       =  3 x ( 1/1x2 + 1/2x3 + 1/3x4 + 1/4x5 + ... + 1/9x10 )\

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

A       =  3 x ( 1 - 1/10 )

A       = 3 x  ( 10/10 - 1/10 )

A       =  3 x 9/10

A       = 27/10

Answer 10

dễ lắm!

rút 3 ra ngoài ta sẽ có 3/2+3/6+3/12+.....+3/90=3*(1/2+1/6+......+1/90)=3*(1/1*2+1/2*3+1/3*4+........+1/9*10)

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

rút gọn rút gọn rút gọn ta sẽ có                                                         =3*(1-1/10)=3*9/10=27/10

Answer 11

9/10,don gian

Answer 12

Theo đề bài ta có:
\(A=\frac{3}{2}+\frac{3}{6}+\frac{3}{12}+\frac{3}{20}+...+\frac{3}{90}\)
\(\Rightarrow A:3=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\)
\(\Rightarrow A:3=\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+...+\frac{1}{9\cdot10}\)
\(\Rightarrow A:3=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}\)
\(\Rightarrow A:3=\frac{1}{1}-\frac{1}{10}\)
\(\Rightarrow A:3=\frac{9}{10}\)
\(\Rightarrow A=\frac{27}{10}\)

Answer 13

tim khoang cach giua cacso  roi tac cac mau ra sau do dung so dau tru so cui la ra