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AIME II

Geometry Difficulty 4.7 AIME Find the answer United States

Let △ABC\triangle ABC be a right triangle with ∠A=90∘\angle A = 90^\circ and BC=38BC = 38. There exist points KK and LL inside the triangle such that
AK=AL=BK=CL=KL=14. AK = AL = BK = CL = KL = 14.
The area of the quadrilateral BKLCBKLC can be expressed as n3n\sqrt{3} for some positive integer nn. Find nn.

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=KLAL = CL = KL, the circumcenter of △ACK\triangle ACK is LL. Similarly, the circumcenter of △ABL\triangle ABL is KK.
Because △AKL\triangle AKL is equilateral, ∠ALK=∠AKL=60∘\angle ALK = \angle AKL = 60^\circ, and it follows that ∠ACK=30∘=∠ABL\angle ACK = 30^\circ = \angle ABL.
Because
∠BAK=∠BAC−∠KAL−∠LAC=30∘−∠LAC=30∘−∠LCA=∠ACK−∠LCA=∠KCL, \angle BAK = \angle BAC - \angle KAL - \angle LAC = 30^\circ - \angle LAC = 30^\circ - \angle LCA = \angle ACK - \angle LCA = \angle KCL,
it follows that the isosceles triangles △ABK\triangle ABK and △CKL\triangle CKL are congruent. Thus AB=CKAB = CK. Because ∠KBL=∠ABL−∠ABK=∠ACK−∠LCK\angle KBL = \angle ABL - \angle ABK = \angle ACK - \angle LCK, it is also true that △BLK≅△ACL\triangle BLK \cong \triangle ACL and AC=BLAC = BL.
It follows by SAS that △ABL≅△KCA\triangle ABL \cong \triangle KCA.
Figure 1
Therefore
Area(BKLC)=Area(△ABC)−Area(△ALK)−Area(△ACL)−Area(△ABK)=Area(△ABC)−Area(△ALK)−Area(△BLK)−Area(△ABK)=Area(△ABC)−Area(△ABL). \begin{align*} \text{Area}(BKLC) &= \text{Area}(\triangle ABC) - \text{Area}(\triangle ALK) - \text{Area}(\triangle ACL) - \text{Area}(\triangle ABK) \\ &= \text{Area}(\triangle ABC) - \text{Area}(\triangle ALK) - \text{Area}(\triangle BLK) - \text{Area}(\triangle ABK) \\ &= \text{Area}(\triangle ABC) - \text{Area}(\triangle ABL). \end{align*}

Set c=AB=CKc = AB = CK and b=AC=BLb = AC = BL. Then
Area(△ABL)=bcsin⁡(∠ABL)2=bc4andArea(△ABC)=bc2, \text{Area}(\triangle ABL) = \frac{bc \sin(\angle ABL)}{2} = \frac{bc}{4} \quad \text{and} \quad \text{Area}(\triangle ABC) = \frac{bc}{2},
from which Area(BKLC)=bc4\text{Area}(BKLC) = \frac{bc}{4}. Applying the Law of Cosines to △ABL\triangle ABL gives
142=AL2=b2+c2−bc3=382−bc3. 14^2 = AL^2 = b^2 + c^2 - bc\sqrt{3} = 38^2 - bc\sqrt{3}.
Thus bc3=382−142=52⋅24bc\sqrt{3} = 38^2 - 14^2 = 52 \cdot 24, implying that Area(BKLC)=52⋅23=1043\text{Area}(BKLC) = 52 \cdot 2\sqrt{3} = 104\sqrt{3}. The requested coefficient of 3\sqrt{3} is 104.

Solution 2

As in the first solution, AC=BLAC = BL. Observe that a 60° rotation around the center of △AKL\triangle AKL moves △AKB\triangle AKB onto △KLC\triangle KLC and △KLB\triangle KLB onto △LAC\triangle LAC. Hence there exists a point PP such that △AKL\triangle AKL and △PBC\triangle PBC are concentric equilateral triangles, as shown below.
Figure 2
Once △AKL\triangle AKL is removed from △PBC\triangle PBC, the area of the quadrilateral BKLCBKLC is 13\frac{1}{3} of the area of the remaining shape. Thus
Area(BKLC)=13(Area(△PBC)−Area(△AKL))=112(382−142)3=1043, \text{Area}(BKLC) = \frac{1}{3}(\text{Area}(\triangle PBC) - \text{Area}(\triangle AKL)) = \frac{1}{12}(38^2 - 14^2)\sqrt{3} = 104\sqrt{3},
as in the first solution.

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