jika jumlah deret geometri 3+9+27+ ... =120 maka jumlah suku nya

a = 3
r = U2/U1 = 9/3 = 3
Sn = a(rⁿ - 1)/(r - 1)
120 = 3(3ⁿ - 1)/(3-1)
120 = 3(3ⁿ - 1)/2
120×2/3 = 3ⁿ - 1
80 = 3ⁿ - 1
3ⁿ = 80+1
3ⁿ = 81
3ⁿ = 3⁴
n = 4