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bilangan terbesar = Un = 168
b = 7
Un = a + (n - 1).b
168 = 84 + (n - 1).7
168 = 84 + 7n - 7
168 = 77 + 7n
7n = 168 - 77
7n = 91
n =
n = 13
Sn = (a + Un)
= (84 + 168)
= (252)
= 13 x 126
= 1638 ......jawaban C
84, 91, 98, 105, ..., 168
Cari banyak sukunya :
Un = a + ( n - 1 ) b
168 = 84 + ( n - 1 ) 7
168 = 84 + 7n - 7
168 = 7n + 77
7n = 91
n = 13 suku
Maka, jumlah semua bilangan kelipatan 7 dari 80 sampai 170 :
Sn = n : 2 ( a + Un )
S₁₃ = 13 : 2 ( 84 + 168 )
= 6,5 × 252
= 1.638