Diketahui suku ke 5 dan suku ke 9 barisan aritmatika masing masing 36 dan 88. Jumlah 42 suku pertama adalah..

Dik U5 = 36
U9 = 88
Dit S42?
Un = a +(n - 1) b
U5 = a +(5 - 1) b = 36
a + 4b = 36

U9 = a +(9 - 1) b = 88
a + 8b = 88

a + 4b = 36
a + 8b = 88
____________ _
-4b = -52
b = -52/-4 = 13

a + 4(13) = 36
a + 52 = 36
a = 36 - 52
a = -16

S42 = n/2 (2.a + (n - 1) b)
= 42/2 ( 2. -16 + (42 - 1) 13)
=42/2 (-32 + 41. 13)
= 42/2 ( -32 + 533)
=42/2 ( 501 )
= 10521