Ta có: A = 6 + 52 + 53 + 54 + ... + 51996 + 51997
A = 1 + 5 + 52 + 53 + ... + 51996 + 51997
5A = 5(1 + 5 + 52 + 53 + ... + 51996 + 51997)
5A = 5 + 52 + 53 + 54 + ... + 51997 + 51998
5A - A = (5 + 52 + 53 + 54 + ... + 51997 + 51998) - (1 + 5 + 52 + 53 + ... + 51996 + 51997)
4A = 51998 - 1
A = \(\frac{5^{1998}-1}{4}\)
A= 6 + 52+ 53+ 54 + ..... + 5 1996+ 51997
=>5A=5+52+53+54+...+51997+51998
=5A-A=(5+52+53+54+...51997+51998)-(1+5+52+53+...+51996+51997)
=4A=51998-1=>A=\(\frac{5^{1998}-1}{4}\)
Vậy ...
hc tốt
\(A=6+5^2+5^3+5^4+....+5^{1996}+5^{1997}\)
\(A=1+5+5^2+5^3+...+5^{1996}+5^{1997}\)
\(5A=5\left(1+5+5^2+5^3+...+5^{1996}+5^{1997}\right)\)
\(5A=5+5^2+5^3+5^4+....+5^{1996}+5^{1997}\)
\(5A-A=\left(5+5^2+5^3+5^4+....+5^{1997}+5^{1998}\right)\)\(-\left(1+5+5^2+5^3+...+5^{1996}+5^{1997}\right)\)
\(4A=5^{1998}-1\)
\(A=\frac{5^{1998}-1}{4}\)
