tolong bantu no 2a menggunakan rumus deret geometri

Bab Barisan dan Deret
Matematika SMP Kelas IX

r = U2 : U1
  = 3³ : 3⁴
  = 1/3

Sn = (a . (1 - rⁿ)) / (1 - r)

S7 = (3⁴ . (1 - (1/3)⁷) / (1 - 1/3)
     = (81 . (2.187/2.187 - 1/2.187) / (3/3 - 1/3)
     = (81 . (2.186/2.187) . 3/2
     =  1.093/9