Googly Eyed Triangle

Geometry Level 4

Point D D is on side B C BC of A B C . \triangle ABC. The incircle ω 1 \omega _1 of A B D \triangle ABD is tangent to B C BC at X X and to A D AD at Y . Y. The incircle ω 2 \omega _2 of A D C \triangle ADC is tangent to B C BC at Z Z and to A D AD at Y . Y. If the radius of ω 1 \omega _1 is 2 , 2, and the radius of ω 2 \omega _2 is 1 , 1, find the value of X Y 2 + Y Z 2 XY^2 + YZ^2 to the nearest tenth.


The answer is 8.

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3 solutions

P and R are the centers, R S P X , Y Q X Z . S R = ( 2 + 1 ) 2 1 2 = 8 , X Q = 2 3 8 , Z Q = 1 3 8 . Y Q = 1 3 P S + S X = 4 3 . X Y 2 + Y Z 2 = ( X Q 2 + Y Q 2 ) + ( Z Q 2 + Y Q 2 ) = 8 \text{P and R are the centers, }~~~~ RS\perp PX,~~ YQ \perp XZ . \\SR=\sqrt{(2+1)^2-1^2}=\sqrt 8,~~\therefore ~XQ=\dfrac 2 3 \sqrt 8 , ~~ZQ=\dfrac 1 3 \sqrt 8.\\YQ=\dfrac 1 3 PS +SX=\dfrac 4 3 .\\XY^2+YZ^2=(XQ^2+YQ^2) + (ZQ^2+YQ^2) =~~~~~\large \color{#D61F06}{8}

Steven Yuan
Feb 1, 2015

Let E E be the center of ω 1 \omega _1 and F F be the center of ω 2 \omega _2 . Let G G be the foot of the perpendicular dropped from F F onto E X . EX. We have F G = X Z FG = XZ because F G X Z FGXZ is a rectangle. Also, E G = 1 EG = 1 and F E = 3. FE = 3. Solving for F G FG using right triangle E F G EFG yields F G = X Z = 8 . FG = XZ = \sqrt{8}.

Now, from Power of a Point, we deduce that X D = D Y = D Z . XD = DY = DZ. This shows that X Y Z XYZ is a right triangle, which can be seen by drawing a circle with center D D passing through X . X. Thus, X Y 2 + Y Z 2 = 8 2 = 8 . XY^2 + YZ^2 = \sqrt{8}^2 = \boxed{8}.

N i c e \color{#D61F06}{Nice} solution. Can you please explain "Power of a point" and equality of three lines? Thanks. I do understand the equality of the three tangent length. it is only with reference of "Power of point" that I need understand.

Niranjan Khanderia - 6 years ago

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Power of a Point states that, given a circle and a point P, and a line passing through P intersecting the circle at points A and B, the product P A ( P B ) PA(PB) is constant. We can use this to show that the tangents are equal.

Steven Yuan - 6 years ago

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Thanks. I did not know the name. I knew this property of the circle. If you do not mind can you please explain how this is applied to show the two tangents to be equal. Thanks.

Niranjan Khanderia - 6 years ago
Atvthe King
Jul 21, 2020

By LoC, note that we have X Y 2 = 2 r 2 + 2 r 2 cos θ XY^2 = 2r^2 + 2r^2 \cos \theta and Y Z 2 = 2 r 2 2 r 2 cos θ YZ^2 = 2r^2-2r^2 \cos \theta , giving their sum to be 4 r 2 4r^2 where r is the tangent length. By Equal Tangents, we note that X D = Z D = r XD = ZD = r . By Pythagoras, we have that X Z = 2 2 XZ = 2\sqrt{2} , giving the answer as 8 8 .

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