WAEC - Mathematics (1989 - No. 2)
If the second and fourth term of a G.P are 8 and 32 respectively,what is the sum of the first four terms?
28
40
48
60
68
설명
ar = 8 .....(1)
ar3 = 32 .....(2)
divide (2) by (1)
r2 = 4 r = 2 ∴ a = 4
S =
a(rn - 1)r - 1
S =
4(24 - 1)2 - 1
= 60