\(A=\frac{9}{1.2}+\frac{9}{2.3}+\frac{9}{3.4}+...+\frac{9}{2019.2020}\)

\(=9\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2019.2020}\right)\)

\(=9\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2019}-\frac{1}{2020}\right)\)

\(=9\left(1-\frac{1}{2020}\right)\)

\(=9.\frac{2019}{2020}\)

\(=\frac{18171}{2020}\)

\(A=\frac{9}{1.2}+\frac{9}{2.3}+\frac{9}{3.4}+...+\frac{9}{2019.2020}\)

\(A=9.\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2019.2020}\right)\)

\(A=9\left(\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2019}-\frac{1}{2020}\right)\)

\(A=9\left(1-\frac{1}{2020}\right)=\frac{9.2019}{2020}=\frac{18171}{2020}\)

...

(11 / 29 + 2 / 29 + 16 / 29 ) . 9 / 10

1 . 9/10 = 9/10

\(\dfrac{9}{10}\times\left(\dfrac{11}{29}+\dfrac{2}{29}+\dfrac{16}{29}\right)=\dfrac{9}{10}\times\dfrac{29}{29}=\dfrac{9}{10}\times1=\dfrac{9}{10}\)

\(\dfrac{4}{7}-\dfrac{4}{9}\)

\(=\dfrac{36}{63}-\dfrac{28}{63}\)

\(=\dfrac{8}{63}\)

các bạn nhớ nha dấu này là dấu / phân số,*là dấu nhân

\(\frac{4}{5}\)x\(\frac{9}{2}\)x\(\frac{2}{9}\)x\(\frac{5}{4}\)

=\(\frac{36}{10}\)x\(\frac{10}{36}\)

=1

thanks HÀ GIANG LÂM nha vì mình cần gấp

Giúp 

a) \(\dfrac{2}{9}+\dfrac{3}{5}-\dfrac{1}{15}=\dfrac{37}{45}-\dfrac{1}{15}=\dfrac{37}{45}-\dfrac{3}{45}=\dfrac{34}{45}\)

b) \(\dfrac{5}{12}\times\dfrac{4}{5}\div4=\dfrac{1}{3}\div4=\dfrac{1}{12}\)

c) \(\dfrac{8}{9}-\dfrac{4}{15}\div\dfrac{2}{5}=\dfrac{8}{9}-\dfrac{2}{3}=\dfrac{8}{9}-\dfrac{6}{9}=\dfrac{2}{9}\)

A= 9+99+999+....+999...9 ( 2021 chữ số 9 )

A= ( 10-1) + ( 100-1 ) + ( 1000-1 ) +...+ (100...00-1) ( có 2021 chữ số 0)

A= 10+100+1000+...+100...00 ( 2021 chữ số 0) - ( 1+1+1+...+1) ( có 2021 chữ số 1)
A= 10=100+1000+...+100.00 ( 2021 chữ số 0)  - 2021

A= 111...10-2021
A=111...09089 ( 2021 chữ số 1)
Bạn ơi k cho mình nha bạn.Mong bạn giữ lời hứa ạ

TL

A= (9+1)+(99+1)+(999+1)+.......+(999....99999+1) - 2021

   =10+100+1000+............+ 1000....000 -2014  ; (1000.....0000 cuối có 2021 chữ số 0)

  = 111111.......1111110 - 2014 ;   (111........111110  cuối có 2021 chữ số 1)

  = 1111.....11111119089   (có 2017 chữ số 1)

Khi nào rảnh vào kênh H-EDITOR xem vid nha!!! Thanks!

có:

(1994-1)+1=1994

Tổng là:

1994x(1994+1):2=1989015

Đáp số:1989015

\(\dfrac{9}{4}-\dfrac{4}{7}:\dfrac{6}{8}=\dfrac{9}{4}-\dfrac{16}{21}=\dfrac{125}{84}\)

\(\dfrac{9}{4}-\dfrac{4}{7}:\dfrac{6}{8}=\dfrac{9}{4}-\dfrac{16}{21}=\dfrac{125}{84}\)