7 的倍數檢查法

eg1: 14
4 x 2 – 1 = 7 (完成)

eg2: 56
6 x 2 – 5 = 7 (完成)

eg3: 154
4 x 2 – 15 = -7 (完成)

eg4: 63
3 x 2 – 6 = 0 (完成)

證明:

假設 x = 10a + b

如果 2b – a = 7m 的話

則 a = 2b – 7m

故 x = 20b – 70m + b = 21b – 70m = 7 (3b – 10m)

Leave a Reply

Your email address will not be published. Required fields are marked *