diketahui u3 =18 dan u5+u8 = 9 tentukan suku ke 31

a + 2b = 18 ---> a = 18 - 2b

(a + 4b) + (a + 7b) = 9
2a + 11b = 9
2(18 - 2b) + 11b = 9
36 - 4b + 11b = 9
-4b + 11b = 9 - 36
7b = -27
b = -27/7

a = 18 - 2b
a = 18 - 2(27/7)
a = 18 - 54/7
a = (126 - 54)/7
a = 72/7

U31 = 72/7 + (30)(-27/7)
       = 72/7 - 810/7
       = -738/7