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Geometry Level 5

Consider Δ A B C \Delta ABC having I I as its incenter and I 1 , I 2 , I 3 I_1,I_2,I_3 as its excenters opposite to vertices A , B , C A,B,C respectively.If Δ A B C \Delta ABC has a unit circumradius , find the value of :

c y c { 1 , 2 , 3 } ( I I 1 2 + I 2 I 3 2 ) \Large\displaystyle\sum_{cyc\{1,2,3\}} (II_1^2 + I_2I_3^2)


The answer is 48.

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2 solutions

Xuming Liang
Aug 10, 2015

Here is a solution without the use of Trignometry. It is well known that I I is the orthocenter of I 1 I 2 I 3 I_1I_2I_3 , and A B C \odot ABC is essentially the nice point circle of I 1 I 2 I 3 I_1I_2I_3 . Therefore we can reword the problem the following way: Given a triangle A B C ABC with orthocenter H H and circumradius 2 2 , find c y c A H 2 + B C 2 \sum_{cyc} AH^2+BC^2 . (Note that half of the circumradius is the radius of the nine point circle.

We will show that A H 2 + B C 2 AH^2+BC^2 is actually constant, given the circumradius is fixed. To see this, let O O denote the circumcenter of A B C ABC , M M is the midpoint of B C BC . It should be well known that 2 O M = A H 2OM=AH , therefore A H 2 + B C 2 = 4 O M 2 + 4 M C 2 = 4 O C 2 = 4 ( 2 ) 2 = 16 AH^2+BC^2=4OM^2+4MC^2=4OC^2=4(2)^2=16 . Hence our answer is 16 3 = 48 16*3=48 .

Moderator note:

Great observation of the change of perspective of the triangle. This is further motivated since the expression only involves I , I 1 , I 2 , I 3 I, I_1, I_2, I_3 .

It's nine point circle not nice \text{nice} point circle!!

Kishore S. Shenoy - 5 years, 9 months ago
Nihar Mahajan
Aug 10, 2015

As per standard trigonometric results we know the following things:

I I 1 = 4 R sin A 2 I I 2 = 4 R sin B 2 I I 1 = 4 R sin C 2 I 1 I 2 = 4 R cos C 2 I 2 I 3 = 4 R cos A 2 I 1 I 3 = 4 R cos B 2 I I 1 2 + I 2 I 3 2 = 16 R 2 sin 2 A 2 + 16 R 2 cos 2 A 2 = 16 R 2 ( sin 2 A 2 + cos 2 A 2 ) = 16 I I 2 2 + I 1 I 3 2 = 16 R 2 sin 2 B 2 + 16 R 2 cos 2 B 2 = 16 R 2 ( sin 2 B 2 + cos 2 B 2 ) = 16 I I 3 2 + I 1 I 2 2 = 16 R 2 sin 2 C 2 + 16 R 2 cos 2 C 2 = 16 R 2 ( sin 2 C 2 + cos 2 C 2 ) = 16 II_1=4R\sin\dfrac{A}{2} \\ II_2=4R\sin\dfrac{B}{2} \\ II_1=4R\sin\dfrac{C}{2} \\ I_1I_2=4R\cos\dfrac{C}{2} \\ I_2I_3=4R\cos\dfrac{A}{2} \\I_1I_3=4R\cos\dfrac{B}{2} \\ II_1^2+I_2I_3^2= 16R^2\sin^2\dfrac{A}{2} + 16R^2\cos^2\dfrac{A}{2}= 16R^2\left(\sin^2\dfrac{A}{2} + \cos^2\dfrac{A}{2}\right) \\ = 16 \\ II_2^2+I_1I_3^2= 16R^2\sin^2\dfrac{B}{2} + 16R^2\cos^2\dfrac{B}{2}= 16R^2\left(\sin^2\dfrac{B}{2} + \cos^2\dfrac{B}{2}\right) \\ = 16 \\ II_3^2+I_1I_2^2= 16R^2\sin^2\dfrac{C}{2} + 16R^2\cos^2\dfrac{C}{2}= 16R^2\left(\sin^2\dfrac{C}{2} + \cos^2\dfrac{C}{2}\right) \\ = 16

So the required sum = 16 + 16 + 16 = 48 \large =16+16+16=\boxed{48}

Instead of saying Trigonometry, say "Using properties of Ex l e \text {Ex}\!\! \bigtriangleup ^ {le} and Pedal l e \text{Pedal}\!\!\bigtriangleup^{le} , we get \cdots "

And also say that I I is the orthocentre of the E x l e I 1 I 2 I 3 Ex\!\! \bigtriangleup^{le}I_1I_2I_3

Kishore S. Shenoy - 5 years, 9 months ago

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