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b=-2
Sn=-1800
Sn=n/2(2a+(n-1)b)
-1800=n/2(2.13+(n-1)-2)
-1800=n/2(26-2n+2)
-1800=n/2(28-2n)
-3600=-2n²+28n
2n²-28n-3600=0
(2n+72)(n-50)=0
n=50
Jadi banyak sukunya adalah 50