Bunuel wrote:
emmak wrote:
For positive integers x and y, which of the following can be written as y^2?
A. (x+1)!
B. (x+9)!
C. x^2−9
D. x^2+1
E. ((x+1)^2)!
A. (x+1)!
B. (x+9)!
C. x^2−9
D. x^2+1
E. ((x+1)^2)!
Plug values: if x=5, then x^2-9=16=4^2.
Answer: C.
Although I understood the explanation, but it would fail if I put x=4. In other words, why did you select x=5?