Question 3.

What is the largest positive integer n such that n2+7n+12n2n12\frac{n^2 + 7n + 12}{n^2 - n - 12} is also a positive integer?

A
6
B
16
C
8
D
12

Question Explanation

Text Explanation

  n2+3n+4n+12n24n+3n12\ \frac{\ n^2+3n+4n+12}{n^2-4n+3n-12}

=  n(n+3)+4(n+3)n(n4)+3(n4)\ \frac{\ n^{ }\left(n+3\right)+4\left(n+3\right)}{n^{ }\left(n-4\right)+3\left(n-4\right)}

= ( n+4)(n+3)(n4)(n+3)\ \frac{\left(\ n+4\right)\left(n+3\right)}{\left(n-4\right)\left(n+3\right)}

= ( n+4)(n4)\ \frac{\left(\ n+4\right)}{\left(n-4\right)}

= ( n4)+8(n4)\ \frac{\left(\ n-4\right)+8}{\left(n-4\right)}

= 1+8(n4)\ 1+\frac{8}{\left(n-4\right)} which will be maximum when n-4 =8

n=12

D is the correct answer.

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