Suku. Ke 7. DaDariri 2, 12, 24... =

Mapel : Matematika
Kelas : IX SMP
Bab : Barisan dan Deret

Pembahasan :
Logika aj itu berpola tambah 2.

2 , 12 , 24

Beda = U2 - U1 = 12 - 2 = 10
Beda = 24 - 12 = 12

Beda t = 12 - 10 = 2 (Tiap pola nambah 2)

Jadi...
2 , 12 , 24 , (24 + 14) , (38 + 16) , (54 + 18) , (72 + 20) , ...

2 , 12 , 24 , 38 , 54 , 72 , 92

Perhatikan bahwa U7 = 92.

2..........12............. 24....
.....10...........12
.............2
a + b + c = 2
3a + b = 10
2a = 2 => a = 2/2 = 1
3a + b = 10 => 3(1) + b = 10 => 3 + b = 10
=> b = 10 -3 = 7
a + b + c = 2 => 1 + 7 + c = 2 => 8 + c = 2
=> c = 2 - 8 = - 6
Un = an^2 + bn + c
Un = 1n^2 + 7n - 6
Un = n^2 + 7n - 6
n = 7
Un = n^2 + 7n - 6
U7 = 7^2 + 7(7) - 6
= 49 + 49 - 6
= 92