Three small circles

Geometry Level 5

In triangle A B C \triangle ABC , let A B = c AB=c , A C = b AC=b and B C = a BC=a . Now, three circles with centers D D , E E and F F are drawn, such that all of them are tangent to the incircle of A B C \triangle ABC . The first one is also tangent to A B AB and A C AC , the second one with A B AB and B C BC , and the third one with A C AC and B C BC .

The radii of these circles are 73 145 36 \dfrac{73-\sqrt{145}}{36} , 66 8 29 25 \dfrac{66-8\sqrt{29}}{25} and 18 8 5 18-8\sqrt{5} , respectively.

If we know that all the sides of the triangle A B C \triangle ABC are integers and a < b < c a<b<c , find 10000 a + 100 b + c 10000a+100b+c .


The answer is 62529.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

First, let the first radius be r a r_a , the second one r b r_b and the third one r c r_c . Also, let r r be the inradius of A B C \triangle ABC . Now, with angle chasing and the general properties of the angle bisector, we get:

r a = r ( 1 sin ( A 2 ) 1 + sin ( A 2 ) ) r_a=r\left(\dfrac{1-\sin(\frac{A}{2})}{1+\sin(\frac{A}{2})}\right) , r b = r ( 1 sin ( B 2 ) 1 + sin ( B 2 ) ) r_b=r\left(\dfrac{1-\sin(\frac{B}{2})}{1+\sin(\frac{B}{2})}\right) , r c = r ( 1 sin ( C 2 ) 1 + sin ( C 2 ) ) r_c=r\left(\dfrac{1-\sin(\frac{C}{2})}{1+\sin(\frac{C}{2})}\right)

Now, we will derive a relationship with r r , r a r_a , r b r_b and r c r_c .

From 1 sin ( θ 2 ) 1 + sin ( θ 2 ) \dfrac{1-\sin(\frac{\theta}{2})}{1+\sin(\frac{\theta}{2})} we get tan 2 ( π θ 4 ) \tan^2 \left(\dfrac{\pi-\theta}{4}\right) with few trigonometric identities.

So, r a = r ( tan 2 ( π A 4 ) ) r_a=r\left(\tan^2 \left(\dfrac{\pi - A}{4}\right)\right) .

Take square root to both sides and get: r a = r tan ( π A 4 ) \sqrt{r_a}=\sqrt{r}\tan\left(\dfrac{\pi - A}{4}\right)

Similarly, r b = r tan ( π B 4 ) \sqrt{r_b}=\sqrt{r}\tan\left(\dfrac{\pi - B}{4}\right) and r c = r tan ( π C 4 ) \sqrt{r_c}=\sqrt{r}\tan\left(\dfrac{\pi - C}{4}\right)

From the identity tan ( A 2 ) tan ( B 2 ) + tan ( A 2 ) tan ( C 2 ) + tan ( B 2 ) tan ( C 2 ) = 1 \tan\left(\dfrac{A}{2}\right)\tan\left(\dfrac{B}{2}\right)+\tan\left(\dfrac{A}{2}\right)\tan\left(\dfrac{C}{2}\right)+\tan\left(\dfrac{B}{2}\right)\tan\left(\dfrac{C}{2}\right)=1 with A + B + C = π A+B+C=\pi , we obtain multiplying the three previous equations, two at time:

r = r a r b + r a r c + r b r c r=\sqrt{r_a*r_b}+\sqrt{r_a*r_c}+\sqrt{r_b*r_c}

Substituting the known values of r a r_a , r b r_b and r c r_c :

r = ( 73 145 36 ) ( 66 8 29 25 ) + ( 73 145 36 ) ( 18 8 5 ) + ( 66 8 29 25 ) ( 18 8 5 ) r=\sqrt{\left(\dfrac{73-\sqrt{145}}{36}\right)\left(\dfrac{66-8\sqrt{29}}{25}\right)}+\sqrt{\left(\dfrac{73-\sqrt{145}}{36}\right)\left(18-8\sqrt{5}\right)}+\sqrt{\left(\dfrac{66-8\sqrt{29}}{25}\right)(18-8\sqrt{5})}

And simplifying a lot, we obtain that r = 2 r=2 .

Next, from the three first equation that we obtained, we're going to solve for the three sines of A B C \triangle ABC :

73 145 36 = 2 ( 1 sin ( A 2 ) 1 + sin ( A 2 ) ) sin ( A 2 ) = 145 145 \dfrac{73-\sqrt{145}}{36}=2\left(\dfrac{1-\sin(\frac{A}{2})}{1+\sin(\frac{A}{2})}\right) \Rightarrow \sin(\frac{A}{2})=\dfrac{\sqrt{145}}{145}

66 8 29 25 = 2 ( 1 sin ( B 2 ) 1 + sin ( B 2 ) ) sin ( B 2 ) = 2 29 29 \dfrac{66-8\sqrt{29}}{25}=2\left(\dfrac{1-\sin(\frac{B}{2})}{1+\sin(\frac{B}{2})}\right) \Rightarrow \sin(\frac{B}{2})=\dfrac{2\sqrt{29}}{29}

18 8 5 = 2 ( 1 sin ( C 2 ) 1 + sin ( C 2 ) ) sin ( C 2 ) = 2 5 5 18-8\sqrt{5}=2\left(\dfrac{1-\sin(\frac{C}{2})}{1+\sin(\frac{C}{2})}\right) \Rightarrow \sin(\frac{C}{2})=\dfrac{2\sqrt{5}}{5}

Obtain the sines with the double angle formula and the Pythagoeran identity:

sin A = 24 145 \sin A=\dfrac{24}{145} , sin B = 20 29 \sin B=\dfrac{20}{29} and sin C = 4 5 \sin C=\dfrac{4}{5}

With the inradius and the sines of the three angles of A B C \triangle ABC , it's easy to obtain every side.

We know that [ A B C ] = a b sin C 2 = a c sin B 2 = b c sin A 2 [ABC]=\dfrac{ab\sin C}{2}=\dfrac{ac\sin B}{2}=\dfrac{bc\sin A}{2} and r = 2 [ A B C ] a + b + c r=\dfrac{2[ABC]}{a+b+c} , so form a equation system:

2 = a b 4 5 a + b + c 2 ( a + b + c ) = 4 a b 5 2=\dfrac{ab *\frac{4}{5}}{a+b+c} \Rightarrow 2(a+b+c)=\dfrac{4ab}{5}

2 = a c 20 29 a + b + c 2 ( a + b + c ) = 20 a c 29 2=\dfrac{ac *\frac{20}{29}}{a+b+c} \Rightarrow 2(a+b+c)=\dfrac{20ac}{29}

2 = b c 24 145 a + b + c 2 ( a + b + c ) = 24 b c 145 2=\dfrac{bc *\frac{24}{145}}{a+b+c} \Rightarrow 2(a+b+c)=\dfrac{24bc}{145}

From the first and the second equation, we get b = 25 c 29 b=\dfrac{25c}{29} ; and from the first and the third equation, we get a = 6 c 29 a=\dfrac{6c}{29} . Substitute them in any equation, let's choose the first one:

2 ( 6 c 29 + 25 c 29 + c ) = 4 ( 6 c 29 ) ( 25 c 29 ) 5 c = 29 2\left(\dfrac{6c}{29}+\dfrac{25c}{29}+c \right)=\dfrac{4(\frac{6c}{29})(\frac{25c}{29})}{5} \Rightarrow c=29

With c c , obtain a a and b b :

a = 6 29 29 a = 6 a=\dfrac{6*29}{29} \Rightarrow a=6

b = 25 29 29 b = 25 b=\dfrac{25*29}{29} \Rightarrow b=25

Finally, 10000 a + 100 b + c = 62529 10000a+100b+c=\boxed{62529} .

If you have an easier and simpler solution, I'd like to see it :D

Alan Enrique Ontiveros Salazar - 6 years, 10 months ago

See my solution.It's much simpler

Ronak Agarwal - 6 years, 10 months ago

Well your solution is great.

Ronak Agarwal - 6 years, 10 months ago

nice solution great

Soummo Mukherjee - 6 years, 5 months ago
Ronak Agarwal
Aug 1, 2014

Firstly I would have also written :

r a = r ( 1 s i n ( A 2 ) 1 + s i n ( A 2 ) ) { r }_{ a }=r(\frac { 1-sin(\frac { A }{ 2 } ) }{ 1+sin(\frac { A }{ 2 } ) } )

r b = r ( 1 s i n ( B 2 ) 1 + s i n ( B 2 ) ) { r }_{ b }=r(\frac { 1-sin(\frac { B }{ 2 } ) }{ 1+sin(\frac { B }{ 2 } ) } )

r c = r ( 1 s i n ( C 2 ) 1 + s i n ( C 2 ) ) { r }_{ c }=r(\frac { 1-sin(\frac { C }{ 2 } ) }{ 1+sin(\frac { C }{ 2 } ) } )

Then I delibrately manipulated your given values like that

r a = 2 ( ( 145 1 ) 2 145 2 1 2 ) = 2 ( 1 1 145 ) ( 1 + 1 145 ) { r }_{ a }=2(\frac { { (\sqrt { 145 } -1) }^{ 2 } }{ { \sqrt { 145 } }^{ 2 }-{ 1 }^{ 2 } } )=2\frac { (1-\frac { 1 }{ \sqrt { 145 } } ) }{ (1+\frac { 1 }{ \sqrt { 145 } } ) }

r b = 2 ( 29 2 ) 2 29 2 2 2 = 2 ( 1 2 29 1 + 2 29 ) { r }_{ b }=2\frac { { (\sqrt { 29 } -2) }^{ 2 } }{ { \sqrt { 29 } }^{ 2 }-{ 2 }^{ 2 } } =2(\frac { 1-\frac { 2 }{ \sqrt { 29 } } }{ 1+\frac { 2 }{ \sqrt { 29 } } } )

r c = 2 ( ( 5 2 ) 2 5 2 2 2 ) = 2 ( 1 2 5 1 + 2 5 ) { r }_{ c }=2(\frac { { (\sqrt { 5 } -2) }^{ 2 } }{ { \sqrt { 5 } }^{ 2 }-{ 2 }^{ 2 } } )=2(\frac { 1-\frac { 2 }{ \sqrt { 5 } } }{ 1+\frac { 2 }{ \sqrt { 5 } } } )

Then it is easy to see that :

r = 2 , s i n ( A 2 ) = 1 145 , s i n ( B 2 ) = 2 29 , s i n ( C 2 ) = 2 5 r=2 \quad ,\quad sin(\frac { A }{ 2 } )=\frac { 1 }{ \sqrt { 145 } } \quad ,\quad sin(\frac { B }{ 2 } )=\frac { 2 }{ \sqrt { 29 } } \quad ,\quad sin(\frac { C }{ 2 } )=\frac { 2 }{ \sqrt { 5 } }

To find the sides use this simple identity

a = r ( c o t B 2 + c o t C 2 ) a=r(cot\frac { B }{ 2 } +cot\frac { C }{ 2 } )

b = r ( c o t A 2 + c o t C 2 ) b=r(cot\frac { A }{ 2 } +cot\frac { C }{ 2 } )

c = r ( c o t A 2 + c o t B 2 ) c=r(cot\frac { A }{ 2 } +cot\frac { B }{ 2 } )

Now c o t A 2 = 12 cot\frac { A }{ 2 } =12 , c o t B 2 = 5 2 cot\frac { B }{ 2 } =\frac { 5 }{ 2 } , c o t C 2 = 1 2 cot\frac { C }{ 2 } =\frac { 1 }{ 2 }

Put the values to get :

a = 6 , b = 29 , c = 25 a=6,b=29,c=25

Hence we have 10000 a + 100 b + c = 62529 \boxed { 10000a+100b+c=62529 }

Brilliant solution, I didn't know that useful identities. With my solution I wanted to give a general solution. You can also try this problem .

Alan Enrique Ontiveros Salazar - 6 years, 10 months ago

I say you should try this problem

Ronak Agarwal - 6 years, 10 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...