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3, 6, 9, 12, 15, ..., 99
Cari banyaknya suku :
Un = a + ( n - 1 ) b
99 = 3 + ( n - 1 ) 3
99 = 3 + 3n - 3
99 = 3n
n = 33 suku
Maka, jumlah semua bilangan itu :
S₃₃ = 33 : 2 ( 3 + 99 )
= 16,5 × 102
= 1.683
bilangan terbesar = Un = 99
b = 3
Un = a + (n - 1).b
99 = 3 + (n - 1).3
99 = 3 + 3n - 3
99 = 3n
n =
n = 33
Sn = .(a + Un)
= .(3 + 99)
= .(102)
= 33.(51)
= 1683