A have the equation x 2 + ( y − 2 ) 2 = 4 and circle B have the equation x 2 + ( y + 1 ) 2 = 9 .
Let circleNow let C 1 be the circle in the first quadrant that is tangent to A , B and the y -axis, and C 2 be the circle in the first quadrant that is tangent to A , B and the x -axis.
If the center of C 1 is at ( x 1 , y 1 ) and the center of C 2 is at ( x 2 , y 2 ) , then find
( y 2 x 2 ) ∗ ( x 1 + 3 y 1 ) .
Clarification: To guarantee a unique solution, both C 1 and C 2 must lie inside B and must both lie entirely within the first quadrant, (inclusive of the x and y axes).
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Nice solution, Krishna; thanks for posting it. Just one typo, though; in your last line, it should be R X rather than X R . :)
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Hey! I found that there is 1 more C 2
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?
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Yes, that triangle would meet all the requirements, except my intent was that both C 1 and C 2 must lie entirely within the first quadrant, (inclusive of the x - and y -axes). I will add another clarification so that my intent is clear; thanks for pointing this out. :)
You have a few typos
r 2 + ( y − 2 ) 2 should be r 2 + ( 2 − y ) 2
and X 2 + ( X − 2 ) 2 should be X 2 + ( 2 − R ) 2
Why do you think the rating has been dropping?
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Is that a typo? They are same
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Oh yeah... crap... But I don't get why it is X − 2 instead of R − 2 or 2 − R for the second one
C 1 has center ( ( 1 2 3 − 2 0 ) , ( 8 − 4 3 ) ) , and C 2 has center ( 2 , 2 1 ) .
Thus ( y 2 x 2 ) ∗ ( x 1 + 3 y 1 ) = ( 2 1 2 ) ∗ ( 1 2 3 − 2 0 + 2 4 − 1 2 3 ) = 4 ∗ 4 = 1 6 .
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 is also tangent to A , B , and the x -axis (and it lies in the first quadrant).
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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.
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No problem! Interestingly, I also had a feeling that there was another circle C 2 that satisfied the conditions, but I couldn't find it. I guess we all have our blind spots.
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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. :)
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...)
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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.
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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.
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@Guiseppi Butel – Oh wow, I didn't see that possibility at all. :)
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@Brian Charlesworth – I am getting a third possible circle for C 1 . Its center is ( 2 5 1 2 6 − 8 , 5 4 + 4 6 ) . It touches A internally and B externally. I think adding the condition that C 1 must also lie inside B will eliminate that case.
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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.
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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 .
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@Pranshu Gaba – Great. Thanks for the feedback. :)
I'm confused by your pink circle and the brown circle. I can't see their connection with the parameters supplied.
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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.
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Ooops. Is the image better now?
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@Calvin Lin – Perfect. Thank you!
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@Brian Charlesworth – Circle B is not represented, to my thinking. Its center is (0,-1) with radius 3.
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@Guiseppi Butel – Alright, I moved it further down so that it's better represented.
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Ok so let me post the solution try to understand without diagram
Solving for C 1
Let centre be (x,y) and radius r
As C 1 touches y-axis, x-coor. of centre = r
C 1 will touch both circles internally so
Distance between centres = difference in radius
r 2 + ( y − 2 ) 2 = 2 − r
r 2 + ( y + 1 ) 2 = 3 − r
Solving these 2 equations we get
r = 1 2 3 − 2 0 , y = 8 − 4 3
r + 3y = 4
Solving for C 2
Let the centre be (X,Y) and radius 'R'
As C 2 touches x-axis, y-coor. = R
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
X 2 + ( R + 1 ) 2 = 3 − R
Solving these 2 equations you will get
R = 2 1 , X = 2
Finally,
R X × ( r + 3 y ) = 1 6