Chọn D

a,

=(10-1)+(10^2   -  1)+...+(10^10  - 1)

=(10 + 10^2 + 10^3 +....+ 10^10) - 10

=10^2+10^3+10^4+....+10^10

=11111111100

b,

1/21+1/28 ko bằng 2/9

Đáp án đúng : B

A=9+99+999+..............999 999 999 999

=(10-1)+(100-1)+(1000-1)+....+(1 000 000 000 000 -1)

=(10+100+1000+...+1 000 000 000 000) - (1 x 12)

=1 111 111 111 110 -12

=1 111 111 098

=>A=1 111 111 098

Bn tham khảo link này nha

https://olm.vn/hoi-dap/detail/10192559186.html

BẠN NÓI CÁCH LÀM VÀ ĐÁP ÁN LUÔN ĐI

\(S1=9+99+999+.....+9999....\)

\(=9^1+9^2+9^3+......+9^{10}\)

\(\Rightarrow9.S1=9^2+9^3+....+9^{11}\)

\(\Rightarrow9S1-S1=\left(9^2+9^3+.....+9^{11}\right)-\left(9^1+9^2+....+9^{10}\right)\)

\(\Rightarrow8S1=9^{11}-9\)

\(\Rightarrow S1=\frac{9^{11}-9}{8}\)

Ta có: Chữ số 9 trong mỗi số tăng dần từ 1,2,3,4,...,2019

Tổng tất cả các chữ số là

(2019-1):1+1=2019(chữ số)

sai

999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999

 A = 9 + 99 + 999 + ... + 999..99 (2019 chữ số 9)

Tổng tất cả các chữ số của A là:

  B = 9 + 9 x 2 + 9 x 3 + 9 x 2019

B = 9 x (1 + 2 + 3+ ... + 2019)

B = 9 x (2019 + 1)x[(2019 - 1) : 1 + 1] : 2

B = 18352710