✔ 最佳答案
1) A three-digit number is formed from the digits 0,1,3,5,6,7,8. Each digit can be used only once. How many possible numbers are even numbers?
There are 7 digits.
Case I, unit digit is 0
There is one choice for unit digit.
There are 6 choices for the tenth digit.
There are 5 choices for the hundred digit.
Together 1 * 6 * 5 = 30 choices
Case II, unit digit is not 0
There are 2 choices for the unit digit (6 or 8)
There are 6 choices for the tenth digit.
There are 4 choices for the hundredth digit (cannot be zero)
Together 2 * 6 * 4 = 48 choices
Case I + Case II = 78 possible numbers
2) Find the number of ways that 8 different flowers can be threaded to form a circular wreath.
Number of ways to thread a linear wreath = 8!
Number of ways to thread a circular wreath = 8! / 8 = 7! = 5040
3) Find the number of diagonals that can be drawn in an n-sided polygon.
Each vertex can join line with each of the other vertices. There are n(n - 1)/2 lines possible.
There are n sides of the polygon to be excluded, so n(n-1)/2 - n = n(n - 3)/2 diagonals
2009-09-13 11:18:41 補充：
(2) I did not realise this is something to consider. Thanks.
(1) The zero is special. It cannot be used in the hundreth position, so I have to split the cases for this special zero.
