Diketahui suku ke 4 dan suku ke 9 suatu deret aritmatika berturut turut adalah 15 dan 30 berapakah jumlah 20 suku pertama deret tersebut

U4 = 15
a + 3b = 15
a = 15-3b

U9 = 30
a + 8b = 30
(15-3b) +8b = 30
15+5b = 30
5b = 15
b = 3

a = 15 -3b
a = 15 -3(3)
a = 15 - 9
a = 6

S20 = 20/2(2(6) + (20-1)3)
S20 = 10(12+57)
S20 = 10(69)
S20 = 690

semoga membantu
ㅡ :)

Dik. Barisan deret aritmatika :
U4 = 15 & U9 = 30.
Dit. S20 ?

Jawab...

u9 = a + 8b = 30
u4 = a + 3b = 15
    ------------------------   (-)
5b = 15
b = 3

a : 
a + 3b = 15
a + 3(3) = 15
a + 9 = 15 
a = 15 - 9
a = 6

Sn = n/2 (2a + (n-1) b)
S20 = 20/2 (2(6) + (19) (3))
S20 = 10 (12 + 57)
S20 = 10 (69)
S20 = 690

Semoga membantu... ^_^