2001 British Mathematical Olympiad
BRITISH MATHEMATICAL OLYMPIAD                                                                 Round 1
          Round 1 : Wednesday, 17 January 2001
                                                                       1. Find all two-digit integers N for which the sum of the digits
Time allowed Three and a half hours.                                      of 10N − N is divisible by 170.
Instructions • Full written solutions - not just answers - are
               required, with complete proofs of any assertions        2. Circle S lies inside circle T and touches it at A. From a
               you may make. Marks awarded will depend on the             point P (distinct from A) on T , chords P Q and P R of T
               clarity of your mathematical presentation. Work            are drawn touching S at X and Y respectively. Show that
               in rough first, and then draft your final version          6 QAR = 26 XAY .
               carefully before writing up your best attempt.
               Do not hand in rough work.
                                                                       3. A tetromino is a figure made up of four unit squares connected
              • One complete solution will gain far more credit           by common edges.
                than several unfinished attempts.    It is more           (i) If we do not distinguish between the possible rotations of
                important to complete a small number of questions              a tetromino within its plane, prove that there are seven
                than to try all five problems.                                 distinct tetrominoes.
              • Each question carries 10 marks.                           (ii) Prove or disprove the statement: It is possible to pack all
              • The use of rulers and compasses is allowed, but                seven distinct tetrominoes into a 4×7 rectangle without
                calculators and protractors are forbidden.                     overlapping.
              • Start each question on a fresh sheet of paper. Write
                on one side of the paper only. On each sheet of        4. Define the sequence (an ) by
                working write the number of the question in the                                                  √
                                                                                                       an = n + { n },
                top left hand corner and your name, initials and
                                                                          where n is a positive integer and {x} denotes the nearest
                school in the top right hand corner.
                                                                          integer to x, where halves are rounded up if necessary.
              • Complete the cover sheet provided and attach it to        Determine the smallest integer k for which the terms
                the front of your script, followed by the questions       ak , ak+1 , . . . , ak+2000 form a sequence of 2001 consecutive
                1,2,3,4,5 in order.                                       integers.
              • Staple all the pages neatly together in the top left
                hand corner.
                                                                       5. A triangle has sides of length a, b, c and its circumcircle has
                                                                          radius R. Prove that the triangle is right-angled if and only
                                                                          if a2 + b2 + c2 = 8R2 .
                            Do not turn over until told to do so.