suku ke n barisan aritmetika dinyatakan dgn Un jika U3 = -2 dan U6=7 tentukan suku ke 13

U6 = a + 5b = 7
U3 = a + 2b = -2
_____________ -
3b = 9
b = 3
a + 5b = 7
a + 5(3) = 7
a + 15 = 7
a = 7 - 15 = -8

U13 = a + (n - 1) b
U13 = -8 + (13 - 1) 3
U13 = -8 + 12 × 3
U13 = -8 + 36
U13 = 28