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b= -8-(-12)= -8+12=4
Un = a+(n-1).b
128=-12+(n-1).4
128=-12+4n-4
128=-16+4n
112=4n
n = 28
banyaknya suku = 28
Sn = 1/2n.(a+Un)
S28=1/2.28.(-12+128)
S28=14.116=1.624
jumlah = 1.624