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U4 = a + 3.b = 19
U7 = a + 6.b = 37
______________-
-3b = -18
b = 6

U4 = a + 3.b = 19
a + 3.6 = 19
a + 18 = 19
a = 19 - 18
a = 1
Masukkan rumus
Sn = n/2 [2a + (n-1)b]
= 24/2 [2.1 + (24-1).6]
= 12 (2 + 23 . 6)
= 12 (2 + 138)
= 12 (140)
Sn = 1680 (B).