jika deret geometri :3+9+27+...=120 maka banyaknya suku adalah.... dg caranya, buat besok nih

jawab
a = 3
r= 3

sn = 120

a (rⁿ  -1) /(r -1) = 120
3 (3ⁿ - 1) / (3-1) = 120
3 (3ⁿ - 1) /2 = 120
3ⁿ - 1 = 120  : (3/2)  
3ⁿ - 1 = 80
3ⁿ = 81 = 3⁴
n = 4

3+9+27+......=120

3 = 1*3
9 = 3*3
27 = 9*3
81 = 27*3

3+9+27+81 = 120

berarti ada 4 suku kak