Pushing the boundaries

Geometry Level 5

Let circle A A have the equation x 2 + ( y 2 ) 2 = 4 x^{2} + (y - 2)^{2} = 4 and circle B B have the equation x 2 + ( y + 1 ) 2 = 9 x^{2} + (y + 1)^{2} = 9 .

Now let C 1 C_{1} be the circle in the first quadrant that is tangent to A , B A, B and the y y -axis, and C 2 C_{2} be the circle in the first quadrant that is tangent to A , B A, B and the x x -axis.

If the center of C 1 C_{1} is at ( x 1 , y 1 ) (x_{1}, y_{1}) and the center of C 2 C_{2} is at ( x 2 , y 2 ) (x_{2}, y_{2}) , then find

( x 2 y 2 ) ( x 1 + 3 y 1 ) (\dfrac{x_{2}}{y_{2}})*(x_{1} + 3y_{1}) .

Clarification: To guarantee a unique solution, both C 1 C_{1} and C 2 C_{2} must lie inside B B and must both lie entirely within the first quadrant, (inclusive of the x x and y y axes).


The answer is 16.

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3 solutions

Krishna Sharma
Nov 27, 2014

Ok so let me post the solution try to understand without diagram

Solving for C 1 C_1

Let centre be (x,y) and radius r r

As C 1 C_1 touches y-axis, x-coor. of centre = r

C 1 C_1 will touch both circles internally so

Distance between centres = difference in radius

r 2 + ( y 2 ) 2 = 2 r \displaystyle \sqrt{r^{2} + (y-2)^{2}} = 2 - r

r 2 + ( y + 1 ) 2 = 3 r \displaystyle \sqrt{r^{2} + (y+1)^{2}} = 3 - r

Solving these 2 equations we get

r = 12 3 20 , y = 8 4 3 r = 12\sqrt{3} -20, y = 8 - 4\sqrt{3}

r + 3y = 4

Solving for C 2 C_2

Let the centre be (X,Y) and radius 'R'

As C 2 C_2 touches x-axis, y-coor. = R

C 2 C_2 will touch the circle with centre(0,2) externally and circle with centre(0,-1) internally

X 2 + ( R 2 ) 2 = 2 + R \displaystyle \sqrt{X^{2} + (R-2)^{2}} = 2 + R

X 2 + ( R + 1 ) 2 = 3 R \displaystyle \sqrt{X^{2} + (R+1)^{2}} = 3 - R

Solving these 2 equations you will get

R = 1 2 , X = 2 R = \frac{1}{2}, X= 2

Finally,

X R × ( r + 3 y ) = 16 \frac{X}{R} \times ( r + 3y) = \boxed{\boxed{16}}

Nice solution, Krishna; thanks for posting it. Just one typo, though; in your last line, it should be X R \frac{X}{R} rather than R X \frac{R}{X} . :)

Brian Charlesworth - 6 years, 6 months ago

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Hey! I found that there is 1 more C 2 C_2

x 2 + ( y 1 ) 2 = 1 \displaystyle x^{2} + (y-1)^{2} = 1

This is tangent to A at origin(x-axis too)

And to B at (0,2)

What do you say?

Krishna Sharma - 6 years, 6 months ago

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Yes, that triangle would meet all the requirements, except my intent was that both C 1 C_{1} and C 2 C_{2} must lie entirely within the first quadrant, (inclusive of the x x - and y y -axes). I will add another clarification so that my intent is clear; thanks for pointing this out. :)

Brian Charlesworth - 6 years, 6 months ago

You have a few typos

r 2 + ( y 2 ) 2 \sqrt { { r }^{ 2 }+{ (y-2) }^{ 2 } } should be r 2 + ( 2 y ) 2 \sqrt { { r }^{ 2 }+{ (2-y) }^{ 2 } }

and X 2 + ( X 2 ) 2 \sqrt { { X }^{ 2 }+{ (X-2) }^{ 2 } } should be X 2 + ( 2 R ) 2 \sqrt { { X }^{ 2 }+{ (2-R) }^{ 2 } }

Why do you think the rating has been dropping?

Julian Poon - 6 years, 6 months ago

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Is that a typo? They are same

Krishna Sharma - 6 years, 6 months ago

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Oh yeah... crap... But I don't get why it is X 2 X-2 instead of R 2 R-2 or 2 R 2-R for the second one

Julian Poon - 6 years, 6 months ago

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@Julian Poon Fixed it :)

Krishna Sharma - 6 years, 6 months ago

C 1 C_{1} has center ( ( 12 3 20 ) , ( 8 4 3 ) ) ((12\sqrt{3} - 20), (8 - 4\sqrt{3})) , and C 2 C_{2} has center ( 2 , 1 2 ) (2, \frac{1}{2}) .

Thus ( x 2 y 2 ) ( x 1 + 3 y 1 ) = ( 2 1 2 ) ( 12 3 20 + 24 12 3 ) = 4 4 = 16 (\dfrac{x_{2}}{y_{2}})*(x_{1} + 3y_{1}) = (\dfrac{2}{\frac{1}{2}})*(12\sqrt{3} - 20 + 24 - 12\sqrt{3}) = 4*4 = \boxed{16} .

I will post the solution method shortly, but for now, here is a diagram of the quartet of circles.

EDIT: I apologize for "pushing the boundaries" of everyone's patience in the refinement of this question. I appreciate all your input. :)

The problem is ambiguous. The circle with equation ( x 4 ) 2 + ( y 2 ) 2 = 4 (x - 4)^2 + (y - 2)^2 = 4 is also tangent to A A , B B , and the x x -axis (and it lies in the first quadrant).

Jon Haussmann - 6 years, 6 months ago

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@Jon Haussmann Thank you! I had a sense that there might be an ambiguity but I just wasn't seeing it; now it's so clear I wonder how I missed it. Thank you also for leaving a comment rather than reporting the question; this is why I like to get some sort of solution posted right away, (even if it's not complete), so that others can catch a mistake like this and let me know directly. Regards.

Brian Charlesworth - 6 years, 6 months ago

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No problem! Interestingly, I also had a feeling that there was another circle C 2 C_2 that satisfied the conditions, but I couldn't find it. I guess we all have our blind spots.

Jon Haussmann - 6 years, 6 months ago

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@Jon Haussmann Haha. Yes, I guess we do. Pranshu Gaba found yet another one, which I hope, given the amended clarification, will be the last. :)

Brian Charlesworth - 6 years, 6 months ago

I missed the point that the whole circle C1 had to be in the first quadrant. I thought that it was sufficient that the centre was in the first quadrant. I consequently got the center of C1 to be (38...., 14...)

Guiseppi Butel - 6 years, 6 months ago

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@Guiseppi Butel Sorry about that. :( When I first wrote the question I had "lying entirely in the first quadrant" in the text, but then shortened it for sake of brevity before posting the question. I guess I should have left it there rather than ending up adding a clarification after the fact. It's often a challenge to avoid ambiguity when posting an original question, particularly in geometry.

Brian Charlesworth - 6 years, 6 months ago

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@Brian Charlesworth I guess I took your title "Pushing the boundaries" too literally. My first center has humungus coordinates (in the 10th power of 10), trying to solve a triangle where the hypotenuse was 2 units greater than value for the x coordinate. After I corrected my error I still got a value greater than 30 for the x coordinate.

Guiseppi Butel - 6 years, 6 months ago

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@Guiseppi Butel Oh wow, I didn't see that possibility at all. :)

Brian Charlesworth - 6 years, 6 months ago

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@Brian Charlesworth I am getting a third possible circle for C 1 C_1 . Its center is ( 12 6 8 25 , 4 + 4 6 5 ) (\frac{12\sqrt{6}-8}{25}, \frac{4+4\sqrt{6}}{5}) . It touches A A internally and B B externally. I think adding the condition that C 1 C_1 must also lie inside B B will eliminate that case.

Pranshu Gaba - 6 years, 6 months ago

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@Pranshu Gaba Sigh.... You're absolutely right. There are circles showing up everywhere. The added condition you have suggested should (finally) clear up any ambiguity, so thank you for that. :)

I see that you solved the question despite the ambiguity. I hope that you didn't end up wasting attempts on the other possibilities.

Brian Charlesworth - 6 years, 6 months ago

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@Brian Charlesworth It wasn't a problem since there was only one case where the final answer was an integer. The question now looks complete to me .

Pranshu Gaba - 6 years, 6 months ago

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@Pranshu Gaba Great. Thanks for the feedback. :)

Brian Charlesworth - 6 years, 6 months ago

I'm confused by your pink circle and the brown circle. I can't see their connection with the parameters supplied.

Guiseppi Butel - 6 years, 6 months ago

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The moderator put that image up without my knowledge, and I agree that it does not represent the parameters of the question. I will ask @Calvin Lin if a more representative image can be posted instead.

Brian Charlesworth - 6 years, 6 months ago

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Ooops. Is the image better now?

Calvin Lin Staff - 6 years, 6 months ago

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@Calvin Lin Perfect. Thank you!

Brian Charlesworth - 6 years, 6 months ago

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@Brian Charlesworth Circle B is not represented, to my thinking. Its center is (0,-1) with radius 3.

Guiseppi Butel - 6 years, 6 months ago

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@Guiseppi Butel Alright, I moved it further down so that it's better represented.

Calvin Lin Staff - 6 years, 6 months ago

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@Calvin Lin Thanks, Calvin.

Guiseppi Butel - 6 years, 6 months ago
Ahmad Saad
Dec 6, 2015

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