Dihitung jumlah 10 suku dari deret aritmatika jika u3 =12 u5= 4

jawab
u3 = 12
u5 = 4

u5 - u3 = 4 - 12
(5-3)b = -8
2b = -8
b=  - 4

u3 = 12
a + 2b = 12
a- 8 = 12
a = 20

s10 = 10/2 ( 2(a) + 9(b)
s10 = 5 (2(20) + 9(-4))
s10= 5  (40 - 36)
s10= 20