bài 1
a) A=5( 1+5+$5^{2}$+$5^{3}$ )+.............+$5^{13}$ .(1+5+$5^{2}$ +$5^{3}$ )
=5.126+...........+$5^{13}$ . 126
= 126.(5+......+ $5^{13}$)
b) A5= $5^{2}$ +$5^{3}$+........+ $5^{17}$
A5-A= ($5^{2}$ +$5^{3}$+........+ $5^{17}$)-(5+$5^{2}$+$5^{3}$+......+ $5^{16}$ )
A4=$5^{17}$-5
⇒ $5^{17}$-5+5=$5^{x}$
⇒ x=17