Jika U3+U7= 57 dan U6+U8= 23 besar suku ke 20 adalah

#u3 + u7 = 57
a + 2b + a + 6b = 57
2a + 8b = 57

#U6 + U8 = 23
a + 5b + a + 7b = 23
2a + 12b = 23

eleminasi

2a + 8b = 57
2a + 12b = 23
----------------------- -
-4b = 34
b = 8.5

2a + 8 (8.5) = 57
2a = 57 - 68
2a = -5.5

=>>>>>

U20 = a + 19b

U20 = - 5.5 + 19 (8.5)

U20 = -5.5 + 161.5

U20 = 156 ===>>> ROCK IT