GeometryDifficulty 8.9ShortlistProve itUnited States
Two incongruent triangles ABC and XYZ are called a pair of pals if they satisfy the following conditions: (a) the two triangles have the same area; (b) let M and W be the respective midpoints of sides BC and YZ. The two sets of lengths {AB,AM,AC} and {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,s are three distinct positive real numbers such that q,r,2s are side lengths of a triangle, then there is an unique triangle PQR with PQ=q,PR=r, and PS=s, where S is the midpoint of side QR. Proof. If q,r,2s are side lengths of a triangle, we construct the unique triangle PRQ1 with PR=r,RQ1=q, and PQ1=2s. Let S be the midpoint of side PQ1. Expend segment RS through S to Q so that PQQ1R is a parallelogram. It is clear that PQR is a triangle with PQ=q,PR=r, and PS=s, where S is the midpoint of side QR. To prove the converse statement, we only need to note that the above procedure can be reversed. □
Lemma 3. Let PQR be a triangle with PQ=q,PR=r, and PS=s, where S is the midpoint of side QR. Then the area of triangle PQR is 412q2r2+8r2s2+8s2q2−q4−r4−16s4. Proof. Because PQQ1R is a parallelogram, triangles PQR and PRQ1 have the same area. Because (PR,RQ1,Q1P)=(r,q,2s), the desired results follows directly from the Heron's formula. □
In view of Lemma 2, if triangles ABC and XYZ are a pair of pals, we may assume without of generality that (AB,AC,AM)=(n,s,t) and (XY,XZ,XW)=(n,t,s). By Lemma 3, we have 2n2s2+8s2t2+8t2n2−n4−s4−16t4=2n2t2+8t2s2+8s2n2−n4−t4−16s4 which simplifies to 6n2t2−6n2s2=15t4−15s4. Because ABC and XYZ are incongruent, we deduce that 2n2=5(t2+s2). We set n=5(k2+(k+1)2)=10k2+10k+1 for some positive integer k. By applying the identity (a2+b2)(c2+d2)=(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+(2k−1)2)(k2+(k+1)2)=(6k2+4k−1)2+(2k2+8k+3)2. We set (n,s,t)=(10k2+10k+1,6k2+4k−1,2k2+8k+3) for positive integer k. For large k, it is easy to see that each of (n,2s,t) and (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) and (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,s are three distinct positive real numbers such that q,r,2s are side lengths of a triangle, then there is an unique triangle PQR with PQ=q,PR=r, and PS=s, where S is the midpoint of side QR.
Proof. If q,r,2s are side lengths of a triangle, we construct the unique triangle PRQ1 with PR=r,RQ1=q, and PQ1=2s. Let S be the midpoint of side PQ1. Expend segment RS through S to Q so that PQQ1R is a parallelogram. It is clear that PQR is a triangle with PQ=q,PR=r, and PS=s, where S is the midpoint of side QR. To prove the converse statement, we only need to note that the above procedure can be reversed. □
Lemma 3. Let PQR be a triangle with PQ=q,PR=r, and PS=s, where S is the midpoint of side QR. Then the area of triangle PQR is 412q2r2+8r2s2+8s2q2−q4−r4−16s4. Proof. Because PQQ1R is a parallelogram, triangles PQR and PRQ1 have the same area. Because (PR,RQ1,Q1P)=(r,q,2s), the desired results follows directly from the Heron's formula. □
In view of Lemma 2, if triangles ABC and XYZ are a pair of pals, we may assume without loss of generality that (AB,AC,AM)=(n,s,t) and (XY,XZ,XW)=(n,t,s). By Lemma 3, we have 2n2s2+8s2t2+8t2n2−n4−s4−16t4=2n2t2+8t2s2+8s2n2−n4−t4−16s4 which simplifies to 6n2t2−6n2s2=15t4−15s4. Because ABC and XYZ are incongruent, we deduce that 2n2=5(t2+s2). We set n=5(k2+(k+1)2)=10k2+10k+1 for some positive integer k. By applying the identity (a2+b2)(c2+d2)=(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+(2k−1)2)(k2+(k+1)2)=(6k2+4k−1)2+(2k2+8k+3)2. We set (n,s,t)=(10k2+10k+1,6k2+4k−1,2k2+8k+3) for positive integer k. For large k, it is easy to see that each of (n,2s,t) and (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) and (n,s,2t) are the side lengths of a pair of pals, completing the solution.
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