Put n = 1
3 - 20 = -17
Put n = 2 ( incremented)
6 - 20 = -14
Put n = 3 ( incremented)
9 - 20 = -11
From the given values deduce a general recurrence relation. The series follow a pattern which is arithmetic sequence pattern.
ao = -17
d = -14 - (-17) = 3
S = n/2 (2ao + d(n-1))
n = 20 -1 + 1 = 20 (as you start from 1 not 2)
S = 20/2( 2 * -17 + 3(20-1)) = 230
