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b= 66-72 = -6
Un= 12
Un= a + (n-1)b = 12
Un= 72 + (n-1)-6 = 12
Un= 72 -6n+6= 12
Un= 78 - 6n =12
-6n= 12-78 = -66
n= -66/-6
n= 11
Sn= n/2 (2a + (n-1)b)
Sn= 11/2 ( 2(72) + (11-1)-6)
Sn= 11/2 ( 144 - 60)
Sn= 11/2 (84)
Sn= 462.
Jadi, banyaknya sukunya adalah 11
dan jumlah deretnya adalah 462