Question

Suku ke-4 dan suku-9 dari deret aritmatika adalah 16 dan 51. jumlah 15 suku yg pertama adalah?...

Answer 1

Un = a + (n-1)b

U4 = a + (4-1)b
= a + 3b
a + 3b = 16

U9 = a + (9-1)b
= a + 8n
a + 8b = 51

a + 8b = 51
a + 3b = 16_-
5b = 35
b = 35 / 5
b = 7

a + 3b = 16
a + 3(7) = 16
a + 21 = 16
a = 16 - 21
a = -5

Sn = 1/2 n (2a + (n-1)b)
S15 = 1/2 . 15 (2(-5) + (15-1)7)
= 1/2 . 15 (-10 + (14)(7))
= 1/2 . 15 (-10 + 98)
= 1/2 . 15 (88)
= 44 . 15
= 660