Haha nice question,
This actually consists of BOTH AP and GP
First you separate the series into 2 parts , the sum of the first 20 terms in (n-1)
i.e (0-1-2-3-....-19) = 20/2(0-19) = -190
then I find the sum of the first 20 terms in 2^n which is
2+ 2^2 +2^3 + ... 2^{20} = 2(2^{20}...