Ta có:
10A=10+102+...........+1099
10A-A=1099+.........+10-1-.........-1098
9A=1099-1
9A=1000.....00 - 1
99 chữ số 0
9A=999......999
99 chữ số 9
A=11111......111
99 chữ số 1
\(A=1+10+10^2+10^3+....+10^{98}.\)
\(\Rightarrow4A=10+10^2+10^3+10^4+...+10^{99}\)
4A - A = ( 10 + 102 + 103 + 104 + ... + 1099) - ( 1 + 10 + 102+ 103+ ......... + 1098)
\(\Rightarrow3A=10^{99}-1\)
\(A=\frac{10^{99}-1}{3}\)
\(A=1+10+10^2+.....+10^{98}.\)
\(\Rightarrow10A=10+10^2+10^3+...+10^{99}\)
\(\Rightarrow10A-A=\left(10+10^2+10^3+...+10^{99}\right)-\left(1+10+10^2+.....+10^{98}\right)\)
\(\Rightarrow9A=10^{99}-1\)
\(\Rightarrow A=\frac{100....00-1}{9}\)( 100 chữ số 0)
\(\Rightarrow A=\frac{99...99}{9}\)( 99 chữ số 9)
\(\Rightarrow A=111.....1\)(99 chữ số 1)
mk lm nhầm :
A = 1 + 10 + 102+ 103 + ... + 1098
=> 10A = 10 + 102+ 103 + 104 + .... + 1099
=> 10A - A = ( 10 + 102+ 103 + 104 + .... + 1099 ) - ( 1 + 10 + 102+ 103 + ... + 1098)
=> 9A = 1099- 1
\(\Rightarrow A=\frac{10^{99}-1}{9}\)