Posted by Elom Charles on Friday, February 17, 2012 at 10:37am.
ar = a+12 r = a/2 a^2/2 = a+12 a^2 - 2a - 24 = 0 (a-6)(a+4) so, a=6, r=3 or, a=-4, r=-2 6, 18, 54, 162, ... or -4, 8, -16, 32, ...