Diketahui barisan aritmatika U15 = 66 dan U7 = 18 tentukan : a. Beda a b. U20 c. S20

U5 = 66
U7 = 17

U5 = a + 4b = 66
U7 = a + 6b = 18
= --------------------(-)
-2b = 48
b = -24
U5 = a + 4 (-24) = 66
= a + (-96) = 66
a = 66 + 96
a = 162
U20 = 162 + 19 (-24)
= 162 -456
= - 294
S20 = n/2 (a + un )