Let R be the radius of the unique (finite) circle that is tangent to the curve y = ∣ x ∣ 1 at precisely two points and to the x -axis at precisely one point.
Find ⌊ 1 0 0 0 0 ∗ R ⌋ .
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My first approach was(very poor) considering circle(in terms of R)
x 2 + y 2 − 2 R y = 0
Substituting y = ∣ x ∣ 1
I got a nasty equation
x 4 ± 2 R x + 1 = 0
Then my next approach was same as yours using tangent and normals :)
Cool problem!
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Thanks. :) Actually, your first approach is just fine if you then substitute x = a and R = a 1 − a 3 , where a > 0 .
I also did exactly same thing...!! But correct in 2nd attempt due calculation mistake in first attempt. Anyway Nice Question Sir as always.
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 .
Consider the first quadrant of the circle where x > 0 , y > 0 . Let the angle made by the radius normal to the y = x 1 with the x-axis as θ .
Then we have x = R cos θ and y = R + R sin θ
We note that the gradient of the curve is given by: d x d y = − x 2 1
And that of the circle is given by: − tan θ 1 .
At the point of contact they are equal. ⇒ x 2 = tan θ .
From x = R cos θ , y = R + R sin θ ⇒ x y = cos θ 1 + sin θ
But x y = x 2 1 = tan θ 1 = sin θ cos θ ⇒ sin θ cos θ = cos θ 1 + sin θ
⇒ sin θ + sin 2 θ = cos 2 θ ⇒ sin θ + sin 2 θ = 1 − sin 2 θ
⇒ 2 sin 2 θ + sin θ − 1 = 0 ⇒ sin θ = 4 − 1 ± ( 1 + 8 ) ,
since θ is acute ⇒ sin θ = 2 1 ⇒ θ = 3 0 o .
Now x = R cos 3 0 o ⇒ x 2 = 4 3 R 2 ⇒ tan 3 0 o = 4 3 R 2 ⇒ R 2 = 3 3 4
⇒ R = 3 4 3 2 = 0 . 8 7 7 3 8 2 6 7 5
And the required answer = ⌊ 0 . 8 7 7 3 8 2 6 7 5 × 1 0 0 0 0 ⌋ = 8 7 7 3
Nice solution. That's interesting that θ comes out to 3 0 degrees; the three normals then divide the circle into three equal sectors. It almost seems like there should be an intuitive reason for this.
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Yes, I actually plotted the curves and circle to check.
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With the radius of the circle being R , by symmetry we know that its center is at ( 0 , R ) .
Now let ( a , a 1 ) be the circle's point of tangency to the curve y = ∣ x ∣ 1 in the first quadrant. Since the tangent line at this point has slope − a 2 1 , the slope of the normal will be a 2 . This normal passes through the center of the circle, and thus has the equation y − R = a 2 x . With ( a , a 1 ) being on this line we thus have that
a 1 − R = a 3 ⟹ R = a 1 − a 3 .
Next, since the distance between the center of the circle and ( a , a 1 ) is R , we have that
R = a 2 + ( a 1 − R ) 2 ⟹ R 2 = a 2 + a 6 ⟹ ( a 1 − a 3 ) 2 = a 2 + a 6
⟹ 1 − 2 a 4 + a 8 = a 4 + a 8 ⟹ a = 3 − 4 1 .
Thus R = a 1 − a 3 = 3 4 1 − 3 − 4 3 = 0 . 8 7 7 3 8 2 6 . . . ,
and so ⌊ 1 0 0 0 0 ∗ R ⌋ = 8 7 7 3 .
Here is a graph of the solution.