Tolong bantuannya Sama caranya ya plissssss

U4 = 13
U4 = a +(4-1)b
13 = a + 3b ... (1)

U9 = 28
U9 = a + (9-1)b
28 = a +8b.... o(2)

eliminasi persamaan 1 dan 2

13 = a + 3b
28 = a + 8b
-------------------- -
-15 = -5b
b = 3

13 = a +3b
13 = a + 9
a = 4

maka,
S20 = 20/2 ( 2(4) + (20-1)3)
S20 = 10(8+57)
S20 = 10(65)
S20 = 650