tolong no 8 ya pakai cara

U4 = 30 U6 = 44
Un = a + (n - 1) b
30 = a + 3b 30 = a + 3b
44 = a + 5b 30 = a + (3.3)
__________ - 30 = a + 9
-6 = -2b a = 21
b = 3

U20 = a + 19b S20 = n/2 (2a + 19b)
= 21 + 57 = 10 (42 + 57)
= 78 = 10 . 99 = 990