Browse · MATH
Printjmc
algebra senior
Problem
Consider two positive even integers less than (not necessarily distinct). When the sum of these two numbers is added to their product, how many different possible values may result?
Solution
Let and be the two integers; then , giving total possible pairs . The question asks for the number of different values of . Notice that by Simon's Favorite Factoring Trick, so it suffices to find the number of different possible values of . Here, .
There are pairs where is equal to ; by symmetry, half of the remaining pairs correspond to swapping the values of and , leaving pairs . Since most of the possible values of and are prime factors that do not divide into any of the other numbers, we note that most of the values of will be distinct. The exception are the numbers divisible by and : ; then, if or , then .
Hence, there are exactly distinct possible values of .
There are pairs where is equal to ; by symmetry, half of the remaining pairs correspond to swapping the values of and , leaving pairs . Since most of the possible values of and are prime factors that do not divide into any of the other numbers, we note that most of the values of will be distinct. The exception are the numbers divisible by and : ; then, if or , then .
Hence, there are exactly distinct possible values of .
Final answer
27