Diketahui barisan 5,12,21,32,..... Suku ke-20 barisan tersebut adalah..... a. 480 b. 450 c. 420 d. 400

maaf ralat ..

a+b+c = 5
3a + b = 7
2a = 2

2a = 2 , a = 1

3a+b = 7
3.1+b = 7 , b = 7-3 = 4

a+b+c = 5
1+4+c = 5 , c = 5-5 = 0

Un= an^2 + bn + C
U20 = 1.20^2 + 4.20
U20 = 400 +80
U20 = 480