suatu barisan aritmatika dimana u3 dan u6 75 sedangkan u8 27 tentukan s8

U3 + U6 = 75
a + 2b + a + 5b = 75
2a + 7b = 75
U8 = a + 7b = 27

2a + 7b = 75
a + 7b = 27
--------------------(-)
a = 48

a + 7b = 27
48 + 7b = 27
7b = 27 - 48
7b = - 21
b = - 3

S8 = ½ . n (2a + (n - 1)b)
= ½ . 8 (2. 48 + (8 - 1)(-3)
= 4 (96 + 7 (-3))
= 4 (96 - 21)
= 4 . 75
= 300