每周数学思维训练题

YIEYIE

1.(Hua 7) Observe that 12 = 1, 112 = 121, 1112 = 12321. For N = 11…12 (10 1’s), find the sum of the digits of N.

2.(CMC 9-98-23/11-98-21)Two numbers 123456789 and 999999999 are multiplied. How many of the digits in the final result are 9’s?

3.(CMC 10-99-12) The numbers 49, 29, 9, 40, 22, 15, 53, 33, 13, 47 are grouped in pairs so that the sum of each pair is the same. Which nu

chrisyue

1. 12=1
     1=1
    112=121
        11
      11
      121
deduce 11111111112=1234567900987654321.
2.123456789*999999999=123456789*1000000000-123456789=1234567887654321.
so the digits of 9=0
3.49+29+9+40+22+15+53+33+13+47=310
310/5=62
62-49 =13
62-29=33
62-9=53
62-40=22