contoh soal + bahasan deret geometri tak hingga ?

Hitung jumlah barisan berikut:
1 + 1/3 + 1/9 + 1/27 + ...
Jawab:
a = 1
r = (1/3)/1 = 1/3
S~ = a/(1 - r)
     = 1/(1 - 1/3)
     = 1/(2/3)
     = 3/2