Jika suku ketiga barisan aritmatika =13 dan U5 + U7 = 50, maka besar suku ke-9 adalah... a. 37 b. 25 c. 15 d. 5 e. 4

13 = a + 2b

U5 + U7 = 50
(a + 4b) + (a+6b) = 50
2a + 10b = 50
a + 5b = 25
U6 = 25

a + 5b = 25
a + 2b = 13
3b = 12
b = 4

a + 2(4) = 13
a = 13-8
a = 5

U9 = 5 + 8(4)
= 5 + 32
= 37 (a.)