Odpowiedź :
Odpowiedź:
Sn = a₁ * [tex]\frac{1-q^{n}}{1-q}[/tex]
aₙ = a₁ * [tex]q^{n-1}[/tex]
q = -5
78125 = -1 * [tex]-5^{n-1}[/tex]
-78125 = [tex]-5^{n-1}[/tex]
-78125 = [tex]-5^{7}[/tex]
[tex]-5^{n-1}[/tex] = [tex]-5^{7}[/tex] -----> n = 8
S₈ = -1 * [tex]\frac{1-(-5)^{8} }{1-(-5)} = -1 * \frac {1-390625}{6} = -1 * (-65104) = 65104[/tex]