jika suku ketiga barisan aritmetika =13 dan U5+U7=50. maka besar suku ke-9 adalah

diketahui: U3 = 13, U5 + U7 = 50, b = beda, a = U1
ditanyakan: U9?
jawab:
U9 = a + 8b
U3 = a + 2b => a + 2b = 13
U5 = a + 4b
U7 = a + 6b

=> U5 + U7 = 50
(a + 4b) + (a + 6b) = 50
2a + 10b = 50

=> a + 2b = 13
2a + 10b = 50

=> 2a + 4b = 26
2a + 10b = 50
------------------------- - (eliminasj)
0 + 6b = 24
b = 24/6
b = 4

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

maka, U9 = a + 8b
U9 = 5 + 8(4)
U9 = 5 + 32
U9 = 37
jadi, besar suku 9 ( U9 ) = 37