Another tight fit .....

Calculus Level 5

Let R R be the radius of the unique (finite) circle that is tangent to the curve y = 1 x y = \dfrac{1}{|x|} at precisely two points and to the x x -axis at precisely one point.

Find 10000 R \lfloor 10000*R \rfloor .


The answer is 8773.

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2 solutions

With the radius of the circle being R R , by symmetry we know that its center is at ( 0 , R ) (0,R) .

Now let ( a , 1 a ) (a, \frac{1}{a}) be the circle's point of tangency to the curve y = 1 x y = \frac{1}{|x|} in the first quadrant. Since the tangent line at this point has slope 1 a 2 -\frac{1}{a^{2}} , the slope of the normal will be a 2 a^{2} . This normal passes through the center of the circle, and thus has the equation y R = a 2 x y - R = a^{2}x . With ( a , 1 a ) (a, \frac{1}{a}) being on this line we thus have that

1 a R = a 3 R = 1 a a 3 \frac{1}{a} - R = a^{3} \Longrightarrow R = \frac{1}{a} - a^{3} .

Next, since the distance between the center of the circle and ( a , 1 a ) (a, \frac{1}{a}) is R R , we have that

R = a 2 + ( 1 a R ) 2 R 2 = a 2 + a 6 ( 1 a a 3 ) 2 = a 2 + a 6 R = \sqrt{a^{2} + (\frac{1}{a} - R)^{2}} \Longrightarrow R^{2} = a^{2} + a^{6} \Longrightarrow (\frac{1}{a} - a^{3})^{2} = a^{2} + a^{6}

1 2 a 4 + a 8 = a 4 + a 8 a = 3 1 4 \Longrightarrow 1 - 2a^{4} + a^{8} = a^{4} + a^{8} \Longrightarrow a = 3^{-\frac{1}{4}} .

Thus R = 1 a a 3 = 3 1 4 3 3 4 = 0.8773826... R = \frac{1}{a} - a^{3} = 3^{\frac{1}{4}} - 3^{-\frac{3}{4}} = 0.8773826... ,

and so 10000 R = 8773 \lfloor 10000*R \rfloor = \boxed{8773} .

Here is a graph of the solution.

My first approach was(very poor) considering circle(in terms of R)

x 2 + y 2 2 R y = 0 \displaystyle x^{2} + y^{2} -2Ry = 0

Substituting y = 1 x y = \frac{1}{|x|}

I got a nasty equation

x 4 ± 2 R x + 1 = 0 x^{4} \pm2Rx +1= 0

Then my next approach was same as yours using tangent and normals :)

Cool problem!

Krishna Sharma - 6 years, 7 months ago

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Thanks. :) Actually, your first approach is just fine if you then substitute x = a x = a and R = 1 a a 3 R = \frac{1}{a} - a^{3} , where a > 0 a \gt 0 .

Brian Charlesworth - 6 years, 7 months ago

I also did exactly same thing...!! But correct in 2nd attempt due calculation mistake in first attempt. Anyway Nice Question Sir as always.

Deepanshu Gupta - 6 years, 7 months ago

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Thank you. :)

Brian Charlesworth - 6 years, 7 months ago
Chew-Seong Cheong
Oct 23, 2014

Due to symmetry, we can see that the circle touches the x-axis at the origin. The equation of the circle is therefore ( y R ) 2 + x 2 = R 2 (y-R)^2 + x^2 = R^2 .

Consider the first quadrant of the circle where x > 0 , y > 0 x > 0, y> 0 . Let the angle made by the radius normal to the y = 1 x y = \dfrac {1}{x} with the x-axis as θ \theta .

Then we have x = R cos θ x = R\cos {\theta} and y = R + R sin θ y = R + R\sin{\theta}

We note that the gradient of the curve is given by: d y d x = 1 x 2 \dfrac {dy}{dx} = - \dfrac {1}{x^2}

And that of the circle is given by: 1 tan θ -\dfrac {1}{\tan{\theta}} .

At the point of contact they are equal. x 2 = tan θ \Rightarrow x^2 = \tan{\theta} .

From x = R cos θ , y = R + R sin θ y x = 1 + sin θ cos θ x = R\cos {\theta}, \quad y = R + R\sin{\theta} \quad \Rightarrow \dfrac {y}{x} = \dfrac {1+\sin{\theta}} {\cos{\theta}}

But y x = 1 x 2 = 1 tan θ = cos θ sin θ cos θ sin θ = 1 + sin θ cos θ \dfrac {y}{x} = \dfrac {1}{x^2} = \dfrac {1}{\tan{\theta}} = \dfrac { \cos {\theta}}{\sin{\theta}}\quad \Rightarrow \dfrac { \cos {\theta}} {\sin{\theta}} = \dfrac {1+ \sin {\theta} } {\cos{\theta}}

sin θ + sin 2 θ = cos 2 θ sin θ + sin 2 θ = 1 sin 2 θ \Rightarrow \sin{\theta}+ \sin^2 {\theta} = \cos^2{\theta}\quad \Rightarrow \sin{\theta}+ \sin^2 {\theta} = 1 - \sin^2{\theta}

2 sin 2 θ + sin θ 1 = 0 sin θ = 1 ± ( 1 + 8 ) 4 \Rightarrow 2\sin^2{\theta}+ \sin{\theta} -1 = 0 \quad \Rightarrow \sin{\theta} = \dfrac {-1 \pm \sqrt(1+8)} {4} ,

since θ \theta is acute sin θ = 1 2 θ = 3 0 o \Rightarrow \sin{\theta} = \frac{1}{2}\quad \Rightarrow \theta = 30^o .

Now x = R cos 3 0 o x 2 = 3 4 R 2 tan 3 0 o = 3 4 R 2 R 2 = 4 3 3 x = R\cos{30^o}\quad \Rightarrow x^2 = \frac {3}{4} R^2\quad \Rightarrow \tan{30^o} = \frac {3}{4} R^2 \quad \Rightarrow R^2 = \frac {4}{3\sqrt{3}}

R = 2 3 3 4 = 0.877382675 \Rightarrow R = \dfrac {2} {3^{\frac{3}{4}}} = 0.877382675

And the required answer = 0.877382675 × 10000 = 8773 = \lfloor 0.877382675 \times 10000 \rfloor = \boxed {8773}

Nice solution. That's interesting that θ \theta comes out to 30 30 degrees; the three normals then divide the circle into three equal sectors. It almost seems like there should be an intuitive reason for this.

Brian Charlesworth - 6 years, 7 months ago

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Yes, I actually plotted the curves and circle to check.

Chew-Seong Cheong - 6 years, 7 months ago

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