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Geometry Difficulty 5.7 AIME, harder Prove it Romania

Given is a regular polygon A1A2...A2010A_1A_2...A_{2010} centered at OO. On each of the segments OAkOA_k, with k=1,2,...,2010k = 1, 2, ..., 2010, lies the point BkB_k such that
OBkOAk=1k. \frac{OB_k}{OA_k} = \frac{1}{k}.
Determine the ratio between the area of the polygon B1B2...B2010B_1B_2...B_{2010} and that of A1A2...A2010A_1A_2...A_{2010}.

Solution

In the sequel [XYZ][XYZ] will represent the area of a triangle XYZXYZ. Denote by SS the area of the polygon A1A2...A2010A_1A_2...A_{2010}.
Notice that [OA1A2]=[OA2A3]==[OA2010A1]=12010S[OA_1A_2] = [OA_2A_3] = \dots = [OA_{2010}A_1] = \frac{1}{2010}S, since the polygon A1A2...A2010A_1A_2...A_{2010} is given as being regular. Then for each k=1,2,...,2009k = 1, 2, ..., 2009 we have
[OBkBk+1][OAkAk+1]=OBkOBk+1OAkOAk+1=1k(k+1), \frac{[OB_k B_{k+1}]}{[OA_k A_{k+1}]} = \frac{OB_k \cdot OB_{k+1}}{OA_k \cdot OA_{k+1}} = \frac{1}{k(k+1)},
as well as
[OB2010B1][OA2010A1]=OB2010OB1OA2010OA1=12010, \frac{[OB_{2010}B_1]}{[OA_{2010}A_1]} = \frac{OB_{2010} \cdot OB_1}{OA_{2010} \cdot OA_1} = \frac{1}{2010},
since the ratio of the areas of two triangles sharing a same angle is equal to the ratio of the products of the corresponding sides.
Therefore denoting by TT the area of the polygon B1B2...B2010B_1B_2...B_{2010}, we have
T=k=12009[OBkBk+1]+[OB2010B1]=S2010(k=120091k(k+1)+12010)=S2010(k=12009(1k1k+1)+12010)=S2010((112010)+12010)=S2010, \begin{align*} T &= \sum_{k=1}^{2009} [OB_k B_{k+1}] + [OB_{2010} B_1] \\ &= \frac{S}{2010} \left( \sum_{k=1}^{2009} \frac{1}{k(k+1)} + \frac{1}{2010} \right) \\ &= \frac{S}{2010} \left( \sum_{k=1}^{2009} \left( \frac{1}{k} - \frac{1}{k+1} \right) + \frac{1}{2010} \right) \\ &= \frac{S}{2010} \left( \left( 1 - \frac{1}{2010} \right) + \frac{1}{2010} \right) = \frac{S}{2010}, \end{align*}
hence TS=12010\frac{T}{S} = \frac{1}{2010}.

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