carilah suku ke 21 dalam suatu barisan aritmatika jika diketahui suku ke 5 dan suku ke 11 adalah 41 dan 23

Un = a + ( n - 1 )b
U5 = a + 4b = 41
U11 = a + 10b = 23
--------------------------(-)
- 6b = 18
b = - 3

a + 4b = 41
a + 4(-3) = 41
a - 12 = 41
a = 41 + 12
a = 53

U21 = a + 20b
= 53 + 20(-3)
= 53 - 60
= - 7

Materi : Barisan dan Deret
Kelas : IX SMP

U5 = U1 + 4b = 41
U11 = U1 + 10b = 23
••••••••••••••••••••••• [-]
=======> -6b = 18
=======> b = -3

U5 = U1 + 4b
41 = U1 + 4(-3)
41 = U1 - 12
U1 = 41 + 12
U1 = 53

Un = U1 + ( n - 1 ) b
U21 = 53 + (21 - 1) (-3)
U21 = 53 + 20 (-3)
U21 = 53 - 60
U21 = -7

Note :
》U1 = a