由 chris8888 » 2008-01-24 00:32
Is the integer N odd?
1. N is divisible by 3.
2. 2N is divisible by twice as many positive integers as N.
=> 2N is divisible by twice as many positive integers as is N.
1). N = 3a insufficient
2). 2N 可以被兩倍於可以除進N一樣多的數所除.
Odd與Even的特性在乘以2以後的因素特性.
15 = 3 * 5 => (1+1)(1+1) = 4個factors
30 = 2 * 3 * 5 => (1+1)(1+1)(1+1) = 8個factors 這裡是關鍵, 導致剛好會變成原來的兩倍多的因子
如果是偶數
6 = 2 * 3 => (1+1)(1+1) = 4個factors
12 = 2^2 * 3 => (2+1)(1+1) = 6個factors 並沒有成為原來的兩倍, 原因在於偶數原來就已經有了number 2, 無法雙倍factors.
sufficient
answer : B