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9 = a + (3-1)b
9 - 2b = a....................(Pers. 1)
U5 = a + (n-1)b
228 = a + (5-1)b
228 = (9-2b) + 4b
219 = 2b
b = 109,5
a = 9-2b = 9 - 219 = -210
Sn = n/2 [2a + (n-1)b]
= n/2 [2.(-210) + (n-1)109,5]
= n/2 (529,5 + 109,5n)
= n (264,75 + 54,75n)
Jadi rumus geometri tersebut adalah Sn = n (264,75 + 54,75n)