soal 3-9-18-30. suku ke 7nya apa?

Materi : Barisan dan deret

Pembahasan :
Aritmatika Tingkat 2
3 , 9 , 18 , 30 , ...

2A = Konstanta
2A = [(U3 - U2) - (U2 - U1)]
2A = [(18 - 9) - (9 - 3)]
2A = [9 - 6]
2A = 3
A = 3/2

3A + B = Beda
3A + B = U2 - U1
3(3/2) + B = 9 - 3
3(3/2) + B = 6
9/2 + B = 6
B = 12/2 - 9/2
B = 3/2

A + B + C = Suku Pertama
3/2 + 3/2 + C = 3
3 + C = 3
C = 0

Ingat Fungsi Kuadrat Kelas VIII
f(x) = Ax² + Bx + C
Un = An² + Bn + C
Un = 3/2n² + 3/2n
Un = 3/2n(n + 1)

U7 = 3/2(7)(7 + 1)
U7 = 3/2(7)(8)
U7 = 84

Un = a/0! + (n-1)b/1! + (n-1)(n-2)c/2!

U7 = 3/1 + (7-1)6/1 + (7²-3(7)+2)3/2
= 3/1 + 36/1 + 90/2
= 3 + 36 + 45
= 84
Maaf kalau salah