For the geometric sequence with tn = 2(-1)^n*(1/3)^n (a) the sum of the first 99 terms (b) the sum of the odd-numbered terms t1 + t3 + t5 +...+ t99 (c) the sum of the even-numbered terms t2 + t4 + t6 +...+ t98 so do i first maybe want to convert that into something simpler? why would they write it like that in the first place. a = -2/3, r = -1/3 so wouldnt tn = (-2/3)(-1/3)^(n-1)? anyways, after doing that, should i turn that equation into Sn = ? how would i do b) and c)?