Question 3977
{{{Sn = a(1-r^n)/(1-r)}}}


need to find r. We know a=4 and that the fourth term, ar^3 = 256 --> r^3 = 64, so r = cube root of 64, so r=4.


So, now: {{{Sn = 4(1-4^4)/(1-4)}}}
{{{Sn = 4(1-256)/(-3)}}}
Sn = 340


this is easy to check in this case, since the GP is small: 4+16+64+256 = 340.


jon.