Diketahui barisan aritmatika, jika U3= 42, U5+U9=52 maka S20=...?

1) U3 = 42
a + 2b = 42

2) U5 + U9 = 52
a + 4b + a + 8b = 52
2a + 12b = 52
2(a + 6b) = 52
a + 6b = 26

a + 2b = 42
a + 6b = 26
---------------- -
-4b = 16
b = -4

a + 2(-4) = 42
a - 8 = 42
a = 50

S20 = 20/2 (2 × 50 + 19(-4))
= 10 (100 - 76)
= 10 × 24
= 240