Hi! If I have a sequence that its first 4 terms are: 30, -31, +32, -32 The pattern is geometric sequence but has alternating signs.. How can I find its sum .. I know it is composed of 2 sequences .. However, when I try to separate the 2 sequences .. I get them of different "lengths" In other words, it is (30+32+34) - (31+33) for 5 terms but for 4 terms I get: In other words, it is (30+32) - (31+33) How can I get general formula for both .. (-1)k helps ! but how! and what about the number of terms Thanks!!!