United Kingdom Mathematics Trust
United Kingdom Mathematics Trust
            British Mathematical Olympiad
            Round 2 : Thursday, 31 January 2008
                                                                                    2007/8 British Mathematical Olympiad
Time allowed Three and a half hours.
               Each question is worth 10 marks.                                                           Round 2
Instructions • Full written solutions - not just answers - are
               required, with complete proofs of any assertions
               you may make. Marks awarded will depend on the
               clarity of your mathematical presentation. Work        1. Find the minimum value of x2 + y 2 + z 2 where x, y, z are real numbers
               in rough first, and then draft your final version
               carefully before writing up your best attempt.                such that x3 + y 3 + z 3 − 3xyz = 1.
               Rough work should be handed in, but should be
               clearly marked.
             • One or two complete solutions will gain far more
               credit than partial attempts at all four problems.     2. Let triangle ABC have incentre I and circumcentre O. Suppose that
             • The use of rulers and compasses is allowed, but           6    AIO = 90◦ and 6 CIO = 45◦ . Find the ratio AB : BC : CA.
               calculators and protractors are forbidden.
             • Staple all the pages neatly together in the top left
               hand corner, with questions 1,2,3,4 in order, and
               the cover sheet at the front.                          3. Adrian has drawn a circle in the xy-plane whose radius is a positive
                                                                         integer at most 2008. The origin lies somewhere inside the circle. You
                 In early March, twenty students will be invited
                                                                         are allowed to ask him questions of the form “Is the point (x, y) inside
                 to attend the training session to be held at
                 Trinity College, Cambridge (3-7 April). At the          your circle?” After each question he will answer truthfully “yes” or
                 training session, students sit a pair of IMO-style      “no”. Show that it is always possible to deduce the radius of the circle
                 papers and 8 students will be selected for further
                 training. Those selected will be expected to            after at most sixty questions. [Note: Any point which lies exactly on
                 participate in correspondence work and to attend        the circle may be considered to lie inside the circle.]
                 further training. The UK Team of 6 for this
                 summer’s International Mathematical Olympiad
                 (to be held in Madrid, Spain 14-22 July) will then
                 be chosen.
                                                                      4. Prove that there are infinitely many pairs of distinct positive integers
                 Do not turn over until told to do so.                   x, y such that x2 + y 3 is divisible by x3 + y 2 .