Explicação passo-a-passo:

[tex] a_{1} = 1[/tex]

[tex]q = 3[/tex]

[tex] a_{8} = a_{1} \: . \: {q}^{7} [/tex]

[tex] a_{8} = 1 \: . \: {3}^{7} [/tex]

[tex] a_{8} = {3}^{7} [/tex]

[tex] a_{8} = 2187[/tex]

[tex] s_{n} = \frac{ a_{1}.( {q}^{n} - 1) }{q - 1} [/tex]

[tex] s_{8} = \frac{1.( {3}^{8} - 1) }{3 - 1} [/tex]

[tex] s_{8} = \frac{ {3}^{8} - 1}{2} [/tex]

[tex] s_{8} = \frac{6561 - 1}{2} [/tex]

[tex] s_{8} = \frac{6560}{2} [/tex]

[tex] s_{8} = 3280[/tex]