In , . The internal bisector of meets at , while the external bisector of meets produced at . If and the lengths of and are integers, how many possible lengths of are there?
HongKong -23 IMO Selection Tests, 2022
Solution
Let the lengths of and be and respectively. By the angle bisector theorem, we have , i.e.
Hence we must choose positive integers for which is a positive integer dividing (note that every such will result in a positive integer value of and the existence of such a figure satisfying all the conditions). As , its only positive factors that are less than are the 12 positive factors of as well as and . Hence there are altogether 16 such values of , and they correspond to 16 possible values of .
Want a route through all this instead of an archive? The track
puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.