Suatu barisan geometri : 40, 200, 1000,..., maka jumlah n suku pertama adalah...

jawab

a = 40
r = 200/40 = 5

sn = a (rⁿ - 1) /(r - 1)

sn = 40. (5ⁿ - 1)/ (5 - 1)

sn = 10. (5ⁿ  - 1)