Question

Calculate the sum of the first five terms of the GP 2,6,18

Answer 1

a=2
d=4
S5 = 5/2 (2a+4d) = _____

Answer 2

easy way: just add them
2+6+18+54+162 = 242

or

a=2 r=3
sum(5) = a(r^5 - 1)/(r-1)
= 2(3^5 - 1)/2
= 242

Answer 3

dang!
I really gotta pay attention to GP vs AP!
or maybe check that d is really constant.

Thanks again, @mathhelper!