I like big circles

Geometry Level 4

Three circles whose radii are 21 cm , 32 cm , 19 cm 21 \text{ cm}, 32 \text{ cm}, 19 \text{ cm} touch one another externally. The tangents at their points of contact meet at a point. Find the distance of the point from the nearest point of contact of the tangents. Give your answer to 3 decimal places.

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The answer is 13.317.

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2 solutions

The key to answering this question correctly is to first of all recognize that the distance of the point of concurrency of tangents P is equidistant from all the 3 points of contact of the circles D , E and F . Since this point is symmetrically located with respect to the points D , E and F , it has to be the incentre of the Δ A B C \Delta ABC .

Therefore the required distance = In radius of the Δ A B C \Delta ABC .

Let S be the semi-perimeter of Δ A B C \Delta ABC ; 2 × S = 21 + 32 + 32 + 19 + 19 + 21 2 \times S = 21 + 32 + 32 + 19 + 19 + 21 S = 72 \Rightarrow S = 72

Area of Δ A B C : Δ \Delta ABC : \Delta = 72 × ( 72 53 ) × ( 72 51 ) × ( 72 40 ) \sqrt{72 \times (72-53) \times (72-51) \times (72-40)}

Inradius = Δ S \frac{ \Delta}{S} (and after putting the values) =13.31665 =13.317 .

Did the same way!

Shivam Hinduja - 6 years, 5 months ago

Instead of putting values

r = Δ s = r 1 r 2 r 3 r 1 + r 2 + r 3 = 13.31665623695878594385358821870552479857505074442762 13.3167 \begin{aligned} r = \dfrac{\Delta}{s} &= \sqrt{\dfrac{r_1r_2r_3}{r_1+r_2+r_3}} \\\\ &=13.31665623695878594385358821870552479857505074442762\\ &\approx 13.3167 \end{aligned}

where r 1 , r 2 r_1,~r_2 and r 3 r_3 are RADII. Thus the answer!

Kishore S. Shenoy - 5 years, 9 months ago

Did the exact same way!

Kishore S. Shenoy - 5 years, 9 months ago

Let P be the meeting point of the tangents.
The three centers of the circles will form a centers triangle.
P, corresponding two points of tangency, and the vertex form a kite.
So the line joining P to the vertex is an angle bisector.
Thus P is the incenter of the triangle.
We therefore desire to find the distance of P from the points of tangency.
This is the inradius r equidistance from all points of tangency .
Centers triangle has sides 19+20=40, 21+32=53, 32+19=51.
r = a r e a s . s = 19 + 21 + 32 = 72. r = 72 32 19 21 72 = 13.3166. P i s a t 13.317 from each point of tangency. \therefore\ r=\dfrac{area} s.\ \ \ \ \ \ \ \ s=19+21+32=72. \\ r=\dfrac {\sqrt {72*32*19*21}}{72}=13.3166.\\ P\ is\ at\ \ \color{#D61F06}{13.317}\ \ \text{from each point of tangency.}



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