Diket: u3 + u9 + u11 = 75 suku tengah barisan tersebut 68. Banyak sukunya 43. Maka u43=

(a + 2b) + (a + 8b) + (a + 10b) = 75
3a + 20b = 75

(43 + 1)/2 = 22

U22 = a + 21b = 68
a = 68 - 21b

3(68 - 21b) + 20b = 75
204 - 63b + 20 b = 75
-43b = 75 - 204
-43b = -129
     b = 3

a = 68 - 21b
a = 68 - 21(3)
a = 68 - 63
a = 5