Question

Diketahui suku ke-7 dari suku ke-10. suatu barisan aritmatika masing-masing 103 dan 88. jumlah 24 suku pertama adalah.

tolong dijawab sama caranya yaa. trims :)

Answer 1

U7 = 103
U10 = 88
n = 24

jadi, u7 = a + 6b = 103
u10= a + 9b = 88 _
-3b = 15
b = -5
a +6b = 103
a + 6(-5) = 103
a + (-30) = 103
a = 103 + 30
a = 133


J24 = 24
------ [2a + (n-1)b]
2
= 12 [2(133) + (24-1)(-5)]
= 12 [266+(-115)]
= 12 * 151
= 1821

insyaallah bener(:

Answer 2

U7 = 103→ a + 8b= 103
U10 = 88→ a+ 11b= 88
pers U7 dan U10 dieliminasi sehingga menghasilkan nilai b
a+8b= 103
a+11=88
----------- -
-3b = 15
b = -5
a+8b =103
a + 8(-5) = 103
a = 103 +40
a= 143
S24 = ...?
Sn = n/2 + ( U1 + Un)
S24 = 24/2 ( 143 + 88)
= 12 (231)
= 2772