Suku ke n yang memenuhi barisan -2,1,4,7,...,40 adalah..

a = -2
b = 4 - 1 = 3

Un = a + ( n - 1) b
40 = -2 + (n - 1)3
40 = -2 + 3n - 3
40 = -5 + 3n
40 + 5 = 3n
45 = 3n
45/3 = n
15 = n