GeometryDifficulty 4.7AIMEFind the answerUnited States
Let △ABC be a right triangle with ∠A=90∘ and BC=38. There exist points K and L inside the triangle such that AK=AL=BK=CL=KL=14. The area of the quadrilateral BKLC can be expressed as n3 for some positive integer n. Find n.
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solutions — 2
Solution 1
Because AL=CL=KL, the circumcenter of △ACK is L. Similarly, the circumcenter of △ABL is K. Because △AKL is equilateral, ∠ALK=∠AKL=60∘, and it follows that ∠ACK=30∘=∠ABL. Because ∠BAK=∠BAC−∠KAL−∠LAC=30∘−∠LAC=30∘−∠LCA=∠ACK−∠LCA=∠KCL, it follows that the isosceles triangles △ABK and △CKL are congruent. Thus AB=CK. Because ∠KBL=∠ABL−∠ABK=∠ACK−∠LCK, it is also true that △BLK≅△ACL and AC=BL. It follows by SAS that △ABL≅△KCA. Therefore Area(BKLC)=Area(△ABC)−Area(△ALK)−Area(△ACL)−Area(△ABK)=Area(△ABC)−Area(△ALK)−Area(△BLK)−Area(△ABK)=Area(△ABC)−Area(△ABL).
Set c=AB=CK and b=AC=BL. Then Area(△ABL)=2bcsin(∠ABL)=4bcandArea(△ABC)=2bc, from which Area(BKLC)=4bc. Applying the Law of Cosines to △ABL gives 142=AL2=b2+c2−bc3=382−bc3. Thus bc3=382−142=52⋅24, implying that Area(BKLC)=52⋅23=1043. The requested coefficient of 3 is 104.
Solution 2
As in the first solution, AC=BL. Observe that a 60° rotation around the center of △AKL moves △AKB onto △KLC and △KLB onto △LAC. Hence there exists a point P such that △AKL and △PBC are concentric equilateral triangles, as shown below. Once △AKL is removed from △PBC, the area of the quadrilateral BKLC is 31 of the area of the remaining shape. Thus Area(BKLC)=31(Area(△PBC)−Area(△AKL))=121(382−142)3=1043, as in the first solution.
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