17,15,13,11..... Urutan ke-21=?

Nilai a = 17
nilai b = 15 - 17 = 13 - 15 = 11 - 13 = -2

Di tanya :

U21??

Penyelesaian :

U21 = a + (n-1)b
U21 = 17 + (20)(-2)
U21 = 17 - 40
U21 = -23

17,15,13,11,9,7,5,3,1,-1,-3, -5,-7, -9,-11.-13,-15,-17,-19,-21,-23