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jmc

algebra senior

Problem

Charlize accidentally omitted two consecutive integers when adding the elements of the arithmetic sequence, . If the sum she obtained is , what is the smallest possible value of ?
Solution
The sum of the arithmetic series is equal to . Let and be the two consecutive integers removed, so that their sum is . It follows that

The smallest numbers that Charlize could have omitted are 1 and 2, so which gives us the inequality . If , then , and if , then , so must be at least 22.

The largest numbers that Charlize could have omitted are and , so which gives us the inequality . If , then , and if , then , so must be at most 23.

From the bounds above, we see that the only possible values of are 22 and 23.

If , then the equation becomes , so . This is impossible, because must be an odd integer.

Therefore, . Note that is possible, because Charlize can omit the numbers 17 and 18 to get the sum .
Final answer
23