求助GMATPREP上数学题

1. if n is a multiple of 5 and n= p²q, where p and q are prim numbers, which of the following must be a multiple of 25?

p²; q² ; pq; p²q²; p³q

the answer is p²q^2,     

why?

2. the rate of a certain chemical reaction is directly proportaional to the square of the concentration of chemical A present and inversely proportiaonl to the concentration of chemical B present. if the concentration of chemical B is increased by 100 percent, which of the following is closest to the percent change in the concentration of chemical A required to keep the rection rate unchanged?

100% decrease; 50% decrease; 40% decrease; 40% increase; 50% increase

the answer is 40% increase, why?

3. if x is an integer, is (x²+1)(x+5) an even number?

1) x is an odd number.

2) each prime factor of x² is greater than 7

the answer is D, 不明白第2个条件。

thank you

http://forum.chasedream.com/dispbbs.asp?boardID=22&ID=156044&page=1

http://forum.chasedream.com/dispbbs.asp?BoardID=22&ID=149128

6楼

（2）因为2也是素数，所以2肯定不是X2的因子，X2是奇数，X2+1是偶数，（X2+1)(X+5)是偶数

1，2都明白了，谢谢你，第3 个题，其实就是说如果 each prime factor of x2 is greater than 2 就是可以成立的，对吗？

嗯，我认为是这样。