(bn) - прогрессия. найти n, если q = 1/3, bn = 6, sn = 726

Sn=(Bn×q-B1)/(q-1)
Bn=B1×q^(n-1)
B1=Bn/q^(n-1)
Sn=(Bn×q-Bn/q^(n-1))/(q-1)
Sn×(q-1)=Bn×q-Bn/q^(n-1)
q^n=Bn×q/(Bn×q-Sn(q-1)
(1/3)^n=6×1/3/(6×1/3-726×(1/3-1))
(1/3)^n=1/243
(1/3)^n=(1/3)^5
n=5