Three circles whose radii are 2 1 cm , 3 2 cm , 1 9 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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Did the same way!
Instead of putting values
r = s Δ = r 1 + r 2 + r 3 r 1 r 2 r 3 = 1 3 . 3 1 6 6 5 6 2 3 6 9 5 8 7 8 5 9 4 3 8 5 3 5 8 8 2 1 8 7 0 5 5 2 4 7 9 8 5 7 5 0 5 0 7 4 4 4 2 7 6 2 ≈ 1 3 . 3 1 6 7
where r 1 , r 2 and r 3 are RADII. Thus the answer!
Did the exact same way!
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
=
s
a
r
e
a
.
s
=
1
9
+
2
1
+
3
2
=
7
2
.
r
=
7
2
7
2
∗
3
2
∗
1
9
∗
2
1
=
1
3
.
3
1
6
6
.
P
i
s
a
t
1
3
.
3
1
7
from each point of tangency.
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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 .
Therefore the required distance = In radius of the Δ A B C .
Let S be the semi-perimeter of Δ A B C ; 2 × S = 2 1 + 3 2 + 3 2 + 1 9 + 1 9 + 2 1 ⇒ S = 7 2
Area of Δ A B C : Δ = 7 2 × ( 7 2 − 5 3 ) × ( 7 2 − 5 1 ) × ( 7 2 − 4 0 )
Inradius = S Δ (and after putting the values) =13.31665 =13.317 .