1986 AIME Problems/Problem 6
The pages of a book are numbered through . When the page numbers of the book were added, one of the page numbers was mistakenly added twice, resulting in an incorrect sum of . What was the number of the page that was added twice?
Denote the page number as , with . The sum formula shows that . Since cannot be very large, disregard it for now and solve . The positive root for . Quickly testing, we find that is too large, but if we plug in we find that our answer is .
Use the same method as above where you represent the sum of integers from to expressed as , plus the additional page number . We can establish an upper and lower bound for the number of pages contained in the book, where the upper bound maximizes the sum of the pages and the lower bound maximizes the extra page (and thus minimizes the total sum of the pages.)
is the quadratic you must solve to obtain the upper bound of and is the quadratic you must solve to obtain the lower bound of .
Solving the two equations gives values that are respectively around and with the quadratic formula, and the only integer between the two is .
This implies that we can plug in and come to the same conclusion as the above solution where .
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