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U2+U4=8
a+b+a+3b = 8
2a+4b = 8
a+2b = 4 ... pers 1
U8 = -6
a+7b = -6 ... pers 2
eliminasi pers 1 dan 2
a+2b = 4
a+7b = -6 _
-5b = 10
b = -2
substitusi
a+2(-2) = 4
a-4 = 4
a = 4+4
a = 8
jumlah suku ke-n
Sn = n/2 (2a+(n-1)b)
S30 = 30/2 (2.8+29(-2))
= 15 (16-58)
= 15 (-42)
= -630