求问一道数学题

琉璃轩轩

x is an interger is (x^2+1)(x+5) an even number?
1. x is an odd number
2. each prime factor of x^2 is greater than 7

求问怎么想第二个部分

jacksontg

prime factor can be 2, 3, 5, 7, 11 ...
so it means the prime factor should be 11, 13 ... onwards
so it means x cannot be even (otherwise the prime factor of x^2 must have 2)
-->x is an odd #

Key: D

琉璃轩轩

jacksontg 发表于 2016-2-26 22:08
prime factor can be 2, 3, 5, 7, 11 ...
so it means the prime factor should be 11, 13 ... onwards
so  ...

谢谢啦