Question

Find S20 for the geometric series 4 + 12 + 36 + 108 + … 5,034,043,187 5,123,498,245` 6,973,568,800 7,321,089,348

Answer 1

a_{1} = 4
a_{2} = 12

a_{2} = a_{1}*q
12 = 4q
q = 3

S20 = a_{1}*
`((q^(20)-1))/((q-1))`

S20 = 4*
`((3^(20)-1))/(3-1)`

S20 = 4*
`(3486784401-1)/(2)`

S20 = 2*3486784400

S20 = 6,973,568,800