CE maths (sum of G.S.)

The sum of the 4th term and the 5th term of a geometric sequence is -4.
ar^3 + ar^4 = -4 (r^3即r 三次)
=> ar^3 ( 1 + r) = -4

the sum of the first two terms is 32
a + ar = 32
=> a(1+r) = 32

so,
r^3 x 32 = -4
=> r^3 = -1/8
=> r = -1/2

2007-01-20 13:03:18 補充：
因r = -1/2a(1-1/2) = 32= a = 64即答案D為正確