Question

1+5+5^2+5^3+...5^119

Answer 1

$s = 1 + 5 + 5 {}^{2} + ... + 5 {}^{119} \\ 5s =5 + 5 {}^{2} + 5 {}^{3} + ... + 5 {}^{120} \\ scadem : \\ 5s - s = 5 {}^{120} - 1 \\ 4s = {5}^{120} - 1 \\ s = \frac{ {5}^{120} - 1 }{4}$