求助道prep数学题~~~谢谢谢谢~~

qwbbjg

If n and y are positive integers and 450y = n³, which of the following must be an integer?



1: y/3*2^2*5
2:y/3^2*2*5
3:y/3*2*5^2



(A) None

(B) I only

(C) II only

(D) III only

(E) I, II, and III

qwbbjg

The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A present and inversely proportional 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 reaction rate unchanged?

(A) 100% decrease
(B) 50% decrease
(C) 40% decrease
(D) 40% increase
(E) 50% increase


还有这个，题目我都看不懂，悲催- -

kazemi

450y=n^3
y=n^3/(3^2*2*5^2)

1:y/(3*2^2*5)=n^3/(3^2*2*5^2)/(3*2^2*5)=n^3/(3^3*2^3*5^3)=(n/(3*2*5))^3 integer
2:y/(3^2*2*5)=n^3/(3^2*2*5^2)/(3^2*2*5)=n^3/(3^4*2^2*5^3) will be an interger when a multiple of 3
3:y/(3*2*5^2)=n^3/(3^2*2*5^2)/(3*2*5^2)=n^3/(3^3*2^2*5^4) will be an interger when a multiple of 5

so 1 only

kazemi

let C=A^2/B

B is increased by 100 percent, mean 2B

C=A^2/B=(2A^2)/2B=(v2A)^2/2B

(v2A-A)/A=(1.4A-A)/A=0.4

so 40% increase for A...