2015 UNCO Math Contest II Answer Key
7) (a) One possible example is a square box with large top and bottom and small height, with the top and bottom colored gold and the four sides blue. If the top and bottom are squares of side , then the total area colored gold is . If the height of the box is , then the total area colored blue is . It is easy to select s and h to make equal to half of . Set or . Choose, say, and .
(b) Triangulate each face by connecting the point of tangency on the face to each vertex of the face. Look at a gold face and one of the triangles into which it has been cut. The gold triangle meets a blue triangle along the edge where the two faces meet. Consider the two triangles and the radii that go from the tangent points on the two faces (=the apexes of the triangles) to the center of the sphere. By symmetry, the two triangles are congruent. This means that for each triangle colored gold, there is a congruent triangle colored blue. Therefore, the total area colored blue is at least as large as the total area colored gold.
9) (a) (b)
10) (a) (b)