Suku ke-2 dan ke-5 barisan aritmatika masimg2 19 dan 31, jumlah 24 suku petama adalah

U2 = 19
U5 = 31
Un = a + (n - 1)b
U2 = a + (2 - 1)b
19 = a + b

U5 = a + (5 - 1)b
31 = a + 4b

a + b = 19
a + 4b = 31
(eliminasi)
-3b = -12
b = 4

(substitusi)
a + b = 19
a + 4 = 19
a = 19 - 4 = 15

S24 = n/2 (2a + (n-1)b)
= 24/2 (2 x 15 + (24 - 1)4)
= 12 (30 + 23 x 4)
= 12 (30 + 92)
= 1464

Mapel : Matematika

Kelas : 9

Bab : Barisan dan Deret Bilangan
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⏩Beda barisan
U5 - U2 = 31 - 19
5b - 2b = 12
3b = 12
b = 12/3
b = 4

⏩Suku Pertama barisan
U2 = 19
a + b = 19
a + 4 = 19
a = 19 - 4
a = 15

Sehingga, Jumlah 24 Suku pertama barisannya :

Sn = n/2 [2a + (n - 1)b]

S24 = 24/2 [2*15 + (24 - 1)(4)]

S24 = 12 [30 + 92]

S24 = 12 x 122

S24 = 1464