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Geometry Difficulty 5.8 AIME, harder Prove it Belarus

Let AA be the sum of all 10 pairwise products of the sides of a convex pentagon, SS be the area of the pentagon.

a) Prove that S15AS \le \frac{1}{5}A.

b) Does there exist a constant c<1/5c < 1/5 such that ScAS \le cA?

(I. Voronovich)

Solution

Answer: b) yes.

We use the following well-known

Lemma. Let a,b,c,da, b, c, d be the lengths of the sides of some quadrilateral, and SS be its area. Then 2Sab+cd2S \le ab + cd and 2Sac+bd2S \le ac + bd.

Let now a,b,c,d,ea, b, c, d, e be the lengths of the sides of the given pentagon, ff be the length of one of its diagonals (see the Fig.), SS be its area. Then 2Sab+cf+de<ab+c(d+e)+de2S \le ab + cf + de < ab + c(d + e) + de, i.e. 2S(ab+cd+de)+ce2S \le (ab + cd + de) + ce. Note that all summands in parentheses are the products of the neighboring sides of the pentagon, while cece is the product of non-neighboring sides.

Summing all similar inequalities which we can obtain by the cyclic permutation abcdeaa \to b \to c \to d \to e \to a, we get

10S3B+C,(1) 10S \le 3B + C, \quad (1)

Figure 1

where BB is the sum of all five pairwise products of the neighboring sides, CC is the sum of all five pairwise products of the non-neighboring sides.

Further, 2Sac+bf+de<ac+b(d+e)+de=(ac+bd+be)+de2S \le ac + bf + de < ac + b(d+e) + de = (ac + bd + be) + de. In the same way we obtain from the last inequality the following one

10S3C+B.(2) 10S \le 3C + B. \quad (2)

Summing (1) and (2) gives the required inequality.

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