Question

Tentukan banyak suku dari barisan berikut
a. 6+9+12+15+…= 756
b. 56+51+46+41+…= -36
c. 10+14+18+22+…=640

Answer 1

Bagian a
a = 6
b = 9 - 6 = 3
Sn = 756

Sn = n/2 (2a + (n - 1) b)
756 = n/2 (2.6 + (n - 1) 3)
756 = n/2 (12 + 3n - 3)
756 = n/2 (9 + 3n)
756 = 4,5n + 1,5n^2
bagi 1,5
504 = 3n + n^2
n^2 + 3n - 504 = 0
(n + 24) (n - 21) = 0
n = -24 V n = 21

n = -24 -> TM

jadi,n = 21



bagian b
a = 56
b = -5

-36 = n/2 (2.56 + (n-1) -5)
-36 = n/2 (112 -5n +5 )
-72 = 117n - 5n^2
5n^2 - 117n - 72 = 0
(5n + 3) (n - 24) = 0
n = - 3/5 V n = 24

n=- 3/5 ->TM

jadi, n=24



bagian c
640 = n/2 (2.10 + (n-1) 4)
640 = n/2 (16 + 4n)
640 = 8n + 2n^2
n^2 + 4n - 320 = 0
(n+20)(n-16) = 0
n=-20 V n=16

n=-20 -> TM

jadi,n= 16





maaf ya kalo salah...