suku kelima sebuah barisan aritmatika adalah -3.Suku kesembilan barisan tersebut adalah 9. Nilai 3U12=...

U5 = -3
U9 = 9

U12???

Jawaban :
U5 - U9 = -3 - 9
4b - 8b = -12
-4b = -12
b = 3

Nilai a:
U5 = a + 4b
-3 = a + 4(3)
-3 - 12 = a
-15 = a

Maka u12:
U12 = a + 11b
U12 = -15 + 11(3)
U12 = -15 + 33
U12 = 18

Maka
3*U12 = 3*18 = 54

- Barisan dan Deret -

U5 = a + 4b
-3 = a + 4b
a = -3 - 4b

U9 = a+ 8b
9 = (-3 - 4b) + 8b
9 = -3 + 4b
4b = 12
b = 3

a = -3-4b
a = -3-4(3)
a = -3-12 = -15

U12 = -15 + (12-1)3
U12 = -15 + 33
U12 = 18

3U12 = 3(18) = 54

semoga membantu
-vin