tentukan suku ke 21 dari barisan aritmatika 17,15,13,11

n1 = 17 selisih 2
n2 = 15 selisih 2
n3 = 13 selisih 2
n4 = 11 selisih 2

nx = 17 - 2(x-1)

jika n21 = 17 - 2(21-1)
= 17 - 40 = -23

17,15,13,11,...
U1 = 17
b = U2-U1 = 15-17 = -2
Un = U1+(n-1)b
U21 = U1+(n-1)b
= 17 + (21-1)-2
= 17 + (20×-2)
= 17+(-40)
= 17-40
= -23