a1=-4, q=1/5, Sn=1/5
Sn= a₁*[1-q(don)/1-q]
1/5 = -4*[ 1-(1/5)(don)/1-1/5]
1/5 = -4*[(1-(1/5)(don)/(-4/5)]/:(-4)
-1/20=[(1-(1/5)(don)*(-5/4)]/:(-5/4)
1/25=[1-(1/5)(don)]
1/25-1=-(1/5)(don)
-24/25=-(1/5)(don)/*(-1)
24/25=1/5(don)
24*(1/5)²-(1/5)(don)=0
podstawmy za (1/5)(don)=t
t= 24/25
zatem 24/25=(1/5)(don)
n =2
an=a₁*q(do n-1)
an=-4*(1/5)=-4/5
