Respostas

2013-07-07T21:21:14-03:00
Use o termo geral da PG:

an=a1.q^(n-1)
a7=a1,q^(7-1)
8=a1.(-2)^(6)
8=a1.64
a1=8/64
a1=1/8
  • PeH
  • Ambicioso
2013-07-07T21:24:43-03:00
Sendo o termo geral da P.G. definido por:

a_n = a_1 \cdot q^{n - 1}

Temos:

a_7 = 8 \\ q = -2

Assim:

a_7 = a_1 \cdot (-2)^{7 - 1} \\\\ 8 = a_1 \cdot (-2)^6 \\\\ 8 = a_1 \cdot 64 \\\\ a_1 = \frac{8}{64} = \boxed{\frac{1}{8}}