If the second and fourth term of a G.P are 8 and 32 respectively,what is the sum of the first four terms?
-
A.
28 -
B.
40 -
C.
48 -
D.
60 -
E.
68
Correct Answer: Option D
Explanation
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