In unit square , points are chosen on side respectively such that is perpendicular to and is perpendicular to . Given that , find all possible values of the length of .
Solution
Let , then since triangles are all similar, we have , therefore it remains to solve the equation We first seek rational solutions for relatively prime positive integers . Therefore we have . Since both and are relatively prime to , we have , so , and it is not difficult to see that is the only integral solution. We can therefore rewrite the original equation as It is not difficult to check that the quadratic factor has no zeroes, therefore is the only solution.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.