Given that the first and forth terms of G.P are 6 and 162 respectively, find the sum of the first three terms of the progression
-
A.
27 -
B.
8 -
C.
78 -
D.
48
Correct Answer: Option C
Explanation
(a=6, U_4 = 162\U_4 = ar^{4-1}\U_4 = ar^3\162 = 6(r)^3\r^3 = 27\r=sqrt[3]{27}\r=3\S_4 = frac{a(r^{n}-1)}{r-1}\=frac{6(3^{3}-1)}{3-1}\=frac{6(27-1)}{2}\=3times26\=78)