Bạn tham khảo :
$A = 2^3+2^6+2^9+.....+2^{90}$
$8A = 2^6 + 2^9 + 2^12 + ... + 2^{93}$
$8A -A =( 2^6 + 2^9 + 2^12 + ... + 2^{93}) - (2^3+2^6+2^9+.....+2^{90})$
$7A = 2^93 - 2^3$
$A = \dfrac{2^{93} - 2^3}{7}$
$B = 5^2+5^4+5^6+.....+5^{80}$
$25B = 5^4 + 5^6 + 5^8 + .. + 5^{82}$
$25B - B = (5^4 + 5^6 + 5^8 + .. + 5^{82}) - ( 5^2+5^4+5^6+.....+5^{80})$
$24B = 5^{82} - 5^2$
$B$ =$\dfrac{5^{82} - 5^2 }{24}$
