Four More Circles in a Box

Geometry Level pending

In rectangle A B D E ABDE , perpendicular segments B C BC and G D GD are drawn in such a way that four incircles can be drawn. If the blue incircles are congruent and the radius of the red incircle is twice the radius of the green incircle, what is the ratio B D A B \frac{BD}{AB} ? If it can be expressed as a b c \frac{a\sqrt b}{c} , where a , b , c a,b,c are positive integers with b b square-free and a , c a,c coprime, submit a + b + c . a+b+c.


The answer is 9.

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2 solutions

In the figure, \MN\parallel BD) and \KL\parallel AB). Denote the lengths of B D BD and A B AB by p p and q q respectively.
Triangles F B D \triangle FBD and F C H \triangle FCH are similar, but since their ratio of similarity equals the ratio of their inradii, which is 1, in fact they are congruent and so are their altitudes to the hypotenuse, i.e. F K = F L = q 2 FK=FL=\frac{q}{2} Likewise, F B G F J D \triangle FBG\sim \vartriangle FJD , thus

F M F N = radius of green circle radius of red circle = 1 2 F M = p 3 and F N = 2 p 3 \begin{aligned} & \frac{FM}{FN}=\frac{\text{radius of green circle}}{\text{radius of red circle}}=\frac{1}{2} \\ & \Rightarrow FM=\frac{p}{3}\ \ \ \text{ and }\ \ \ FN=\frac{2p}{3} \\ \end{aligned} On right triangle F B D \triangle FBD ,

F K 2 = K B K D F K 2 = F M F N ( q 2 ) 2 = p 3 2 p 3 p 2 q 2 = 9 8 p q = 3 2 2 = 3 2 4 \begin{aligned} F{{K}^{2}}=KB\cdot KD & \Rightarrow F{{K}^{2}}=FM\cdot FN \\ & \Rightarrow {{\left( \dfrac{q}{2} \right)}^{2}}=\frac{p}{3}\cdot \dfrac{2p}{3} \\ & \Rightarrow \dfrac{{{p}^{2}}}{{{q}^{2}}}=\dfrac{9}{8} \\ & \Rightarrow \frac{p}{q}=\dfrac{3}{2\sqrt{2}}=\dfrac{3\sqrt{2}}{4} \\ \end{aligned} Hence, a + b + c = 3 + 2 + 4 = 9 a+b+c=3+2+4=\boxed{9} .

David Vreken
Dec 23, 2020

Extend G D GD and A E AE to meet at H H , and extend B C BC and D E DE to meet at I I , and label the diagram as follows:

By alternate interior angles and vertical angles, F H C F D B \triangle FHC \sim \triangle FDB by AA similarity, and since their incircles are congruent, F H C F D B \triangle FHC \cong \triangle FDB , and L F = F M LF = FM .

Likewise, F B G F I D \triangle FBG \sim \triangle FID by AA similarity, and since the radius of the red circle is twice the radius of the green circle, F K = 2 J F FK = 2 \cdot JF , which means L D = 2 B L LD = 2 \cdot BL .

L F B L D F \triangle LFB \sim \triangle LDF by AA similarity, so B L L F = L F L D \cfrac{BL}{LF} = \cfrac{LF}{LD} , or B L L F = L F 2 B L \cfrac{BL}{LF} = \cfrac{LF}{2 \cdot BL} , which solves to L F = 2 B L LF = \sqrt{2} \cdot BL .

Therefore, the ratio B D A B = B L + L D 2 L F = B L + 2 B L 2 2 B L = 3 2 2 = 3 2 4 \cfrac{BD}{AB} = \cfrac{BL + LD}{2 \cdot LF} = \cfrac{BL + 2 \cdot BL}{2 \cdot \sqrt{2} \cdot BL} = \cfrac{3}{2\sqrt{2}} = \cfrac{3\sqrt{2}}{4} , so that a = 3 a = 3 , b = 2 b = 2 , c = 4 c = 4 , and a + b + c = 9 a + b + c = \boxed{9} .

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