✔ 最佳答案
S = 2 + 3 + (9/2) + (27/4) + ..... + (2187/64)
S = a + ar + ar² + ar³ + ............
a = 2
r = ar / a = 3/2
2014-03-16 21:02:50 補充:
(2187/64) / (2) = r^(n-1) = (3/2)^(n-1)
(2187/128) = (3/2)^(n-1)
log(2187/128) / log(3/2) = n-1
n-1=7
n=8
2014-03-16 21:08:49 補充:
S(n) = a(1-r^n)/(1-r)
=(2)[1-(3/2)^8]/[1-(3/2)]
=(2)[1-6561/256] / [-1/2]
=(2)[-6305/256] / [-1/2]
=6305 / 64