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Geometry Difficulty 8.9 Shortlist Prove it United States

Two incongruent triangles ABCABC and XYZXYZ are called a pair of pals if they satisfy the following conditions:
(a) the two triangles have the same area;
(b) let MM and WW be the respective midpoints of sides BCBC and YZYZ. The two sets of lengths {AB,AM,AC}\{AB, AM, AC\} and {XY,XW,XZ}\{XY, XW, XZ\} are identical 3-element sets of pairwise relatively prime integers.

Determine if there are infinitely many pairs of triangles that are pals of each other.

Solutions — 2

Solution 1

Lemma 2. The following statement and its converse are both true: If q,r,sq, r, s are three distinct positive real numbers such that q,r,2sq, r, 2s are side lengths of a triangle, then there is an unique triangle PQRPQR with PQ=q,PR=rPQ = q, PR = r, and PS=sPS = s, where SS is the midpoint of side QRQR.
Proof. If q,r,2sq, r, 2s are side lengths of a triangle, we construct the unique triangle PRQ1PRQ_1 with PR=r,RQ1=qPR = r, RQ_1 = q, and PQ1=2sPQ_1 = 2s. Let SS be the midpoint of side PQ1PQ_1. Expend segment RSRS through SS to QQ so that PQQ1RPQQ_1R is a parallelogram. It is clear that PQRPQR is a triangle with PQ=q,PR=rPQ = q, PR = r, and PS=sPS = s, where SS is the midpoint of side QRQR. To prove the converse statement, we only need to note that the above procedure can be reversed. □

Lemma 3. Let PQRPQR be a triangle with PQ=q,PR=rPQ = q, PR = r, and PS=sPS = s, where SS is the midpoint of side QRQR. Then the area of triangle PQRPQR is
142q2r2+8r2s2+8s2q2q4r416s4. \frac{1}{4}\sqrt{2q^2r^2 + 8r^2s^2 + 8s^2q^2 - q^4 - r^4 - 16s^4}.
Proof. Because PQQ1RPQQ_1R is a parallelogram, triangles PQRPQR and PRQ1PRQ_1 have the same area. Because (PR,RQ1,Q1P)=(r,q,2s)(PR, RQ_1, Q_1P) = (r, q, 2s), the desired results follows directly from the Heron's formula. □

In view of Lemma 2, if triangles ABCABC and XYZXYZ are a pair of pals, we may assume without of generality that (AB,AC,AM)=(n,s,t)(AB, AC, AM) = (n, s, t) and (XY,XZ,XW)=(n,t,s)(XY, XZ, XW) = (n, t, s). By Lemma 3, we have
2n2s2+8s2t2+8t2n2n4s416t4=2n2t2+8t2s2+8s2n2n4t416s4 2n^2s^2 + 8s^2t^2 + 8t^2n^2 - n^4 - s^4 - 16t^4 = 2n^2t^2 + 8t^2s^2 + 8s^2n^2 - n^4 - t^4 - 16s^4
which simplifies to
6n2t26n2s2=15t415s4. 6n^2t^2 - 6n^2s^2 = 15t^4 - 15s^4.
Because ABCABC and XYZXYZ are incongruent, we deduce that 2n2=5(t2+s2)2n^2 = 5(t^2 + s^2). We set n=5(k2+(k+1)2)=10k2+10k+1n = 5(k^2 + (k+1)^2) = 10k^2 + 10k + 1 for some positive integer kk. By applying the identity (a2+b2)(c2+d2)=(ac+bd)2(adbc)2(a^2 + b^2)(c^2 + d^2) = (ac + bd)^2 - (ad - bc)^2 repeatedly, we have
t2+s2=10n2=(12+32)(k2+(k+1)2)(k2+(k+1)2)=((4k+3)2+(2k1)2)(k2+(k+1)2)=(6k2+4k1)2+(2k2+8k+3)2. \begin{aligned} t^2 + s^2 &= 10n^2 = (1^2 + 3^2)(k^2 + (k+1)^2)(k^2 + (k+1)^2) \\ &= ((4k+3)^2 + (2k-1)^2)(k^2 + (k+1)^2) \\ &= (6k^2 + 4k - 1)^2 + (2k^2 + 8k + 3)^2. \end{aligned}
We set (n,s,t)=(10k2+10k+1,6k2+4k1,2k2+8k+3)(n, s, t) = (10k^2 + 10k + 1, 6k^2 + 4k - 1, 2k^2 + 8k + 3) for positive integer kk. For large kk, it is easy to see that each of (n,2s,t)(n, 2s, t) and (n,s,2t)(n, s, 2t) is a set of side lengths of a triangle. Furthermore, by the Euclidean algorithm, it is not difficult to check these numbers are pairwise relatively prime to each other, hence (n,2s,t)(n, 2s, t) and (n,s,2t)(n, s, 2t) are the side lengths of a pair of pals, completing the solution.

Solution 2

The answer is yes.
We start with the following observations.

Lemma 2. The following statement and its converse are both true: If q,r,sq, r, s are three distinct positive real numbers such that q,r,2sq, r, 2s are side lengths of a triangle, then there is an unique triangle PQRPQR with PQ=q,PR=rPQ = q, PR = r, and PS=sPS = s, where SS is the midpoint of side QRQR.

Proof. If q,r,2sq, r, 2s are side lengths of a triangle, we construct the unique triangle PRQ1PRQ_1 with PR=r,RQ1=qPR = r, RQ_1 = q, and PQ1=2sPQ_1 = 2s. Let SS be the midpoint of side PQ1PQ_1. Expend segment RSRS through SS to QQ so that PQQ1RPQQ_1R is a parallelogram. It is clear that PQRPQR is a triangle with PQ=q,PR=rPQ = q, PR = r, and PS=sPS = s, where SS is the midpoint of side QRQR. To prove the converse statement, we only need to note that the above procedure can be reversed. □

Lemma 3. Let PQRPQR be a triangle with PQ=q,PR=rPQ = q, PR = r, and PS=sPS = s, where SS is the midpoint of side QRQR. Then the area of triangle PQRPQR is
142q2r2+8r2s2+8s2q2q4r416s4. \frac{1}{4}\sqrt{2q^2r^2 + 8r^2s^2 + 8s^2q^2 - q^4 - r^4 - 16s^4}.
Proof. Because PQQ1RPQQ_1R is a parallelogram, triangles PQRPQR and PRQ1PRQ_1 have the same area. Because (PR,RQ1,Q1P)=(r,q,2s)(PR, RQ_1, Q_1P) = (r, q, 2s), the desired results follows directly from the Heron's formula. □

In view of Lemma 2, if triangles ABCABC and XYZXYZ are a pair of pals, we may assume without loss of generality that (AB,AC,AM)=(n,s,t)(AB, AC, AM) = (n, s, t) and (XY,XZ,XW)=(n,t,s)(XY, XZ, XW) = (n, t, s). By Lemma 3, we have
2n2s2+8s2t2+8t2n2n4s416t4=2n2t2+8t2s2+8s2n2n4t416s4 2n^2s^2 + 8s^2t^2 + 8t^2n^2 - n^4 - s^4 - 16t^4 = 2n^2t^2 + 8t^2s^2 + 8s^2n^2 - n^4 - t^4 - 16s^4
which simplifies to
6n2t26n2s2=15t415s4. 6n^2t^2 - 6n^2s^2 = 15t^4 - 15s^4.
Because ABCABC and XYZXYZ are incongruent, we deduce that 2n2=5(t2+s2)2n^2 = 5(t^2 + s^2). We set n=5(k2+(k+1)2)=10k2+10k+1n = 5(k^2 + (k+1)^2) = 10k^2 + 10k + 1 for some positive integer kk. By applying the identity (a2+b2)(c2+d2)=(ac+bd)2(adbc)2(a^2 + b^2)(c^2 + d^2) = (ac + bd)^2 - (ad - bc)^2 repeatedly, we have
t2+s2=10n2=(12+32)(k2+(k+1)2)(k2+(k+1)2)=((4k+3)2+(2k1)2)(k2+(k+1)2)=(6k2+4k1)2+(2k2+8k+3)2. \begin{aligned} t^2 + s^2 &= 10n^2 = (1^2 + 3^2)(k^2 + (k+1)^2)(k^2 + (k+1)^2) \\ &= ((4k+3)^2 + (2k-1)^2)(k^2 + (k+1)^2) \\ &= (6k^2 + 4k - 1)^2 + (2k^2 + 8k + 3)^2. \end{aligned}
We set (n,s,t)=(10k2+10k+1,6k2+4k1,2k2+8k+3)(n, s, t) = (10k^2 + 10k + 1, 6k^2 + 4k - 1, 2k^2 + 8k + 3) for positive integer kk. For large kk, it is easy to see that each of (n,2s,t)(n, 2s, t) and (n,s,2t)(n, s, 2t) is a set of side lengths of a triangle. Furthermore, by the Euclidean algorithm, it is not difficult to check these numbers are pairwise relatively prime to each other, hence (n,2s,t)(n, 2s, t) and (n,s,2t)(n, s, 2t) are the side lengths of a pair of pals, completing the solution.

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