△ A B C , let A B = c , A C = b and B C = a . Now, three circles with centers D , E and F are drawn, such that all of them are tangent to the incircle of △ A B C . The first one is also tangent to A B and A C , the second one with A B and B C , and the third one with A C and B C .
In triangleThe radii of these circles are 3 6 7 3 − 1 4 5 , 2 5 6 6 − 8 2 9 and 1 8 − 8 5 , respectively.
If we know that all the sides of the triangle △ A B C are integers and a < b < c , find 1 0 0 0 0 a + 1 0 0 b + c .
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If you have an easier and simpler solution, I'd like to see it :D
See my solution.It's much simpler
Well your solution is great.
nice solution great
Firstly I would have also written :
r a = r ( 1 + s i n ( 2 A ) 1 − s i n ( 2 A ) )
r b = r ( 1 + s i n ( 2 B ) 1 − s i n ( 2 B ) )
r c = r ( 1 + s i n ( 2 C ) 1 − s i n ( 2 C ) )
Then I delibrately manipulated your given values like that
r a = 2 ( 1 4 5 2 − 1 2 ( 1 4 5 − 1 ) 2 ) = 2 ( 1 + 1 4 5 1 ) ( 1 − 1 4 5 1 )
r b = 2 2 9 2 − 2 2 ( 2 9 − 2 ) 2 = 2 ( 1 + 2 9 2 1 − 2 9 2 )
r c = 2 ( 5 2 − 2 2 ( 5 − 2 ) 2 ) = 2 ( 1 + 5 2 1 − 5 2 )
Then it is easy to see that :
r = 2 , s i n ( 2 A ) = 1 4 5 1 , s i n ( 2 B ) = 2 9 2 , s i n ( 2 C ) = 5 2
To find the sides use this simple identity
a = r ( c o t 2 B + c o t 2 C )
b = r ( c o t 2 A + c o t 2 C )
c = r ( c o t 2 A + c o t 2 B )
Now c o t 2 A = 1 2 , c o t 2 B = 2 5 , c o t 2 C = 2 1
Put the values to get :
a = 6 , b = 2 9 , c = 2 5
Hence we have 1 0 0 0 0 a + 1 0 0 b + c = 6 2 5 2 9
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 .
I say you should try this problem
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First, let the first radius be r a , the second one r b and the third one r c . Also, let r be the inradius of △ A B C . Now, with angle chasing and the general properties of the angle bisector, we get:
r a = r ( 1 + sin ( 2 A ) 1 − sin ( 2 A ) ) , r b = r ( 1 + sin ( 2 B ) 1 − sin ( 2 B ) ) , r c = r ( 1 + sin ( 2 C ) 1 − sin ( 2 C ) )
Now, we will derive a relationship with r , r a , r b and r c .
From 1 + sin ( 2 θ ) 1 − sin ( 2 θ ) we get tan 2 ( 4 π − θ ) with few trigonometric identities.
So, r a = r ( tan 2 ( 4 π − A ) ) .
Take square root to both sides and get: r a = r tan ( 4 π − A )
Similarly, r b = r tan ( 4 π − B ) and r c = r tan ( 4 π − C )
From the identity tan ( 2 A ) tan ( 2 B ) + tan ( 2 A ) tan ( 2 C ) + tan ( 2 B ) tan ( 2 C ) = 1 with A + B + C = π , we obtain multiplying the three previous equations, two at time:
r = r a ∗ r b + r a ∗ r c + r b ∗ r c
Substituting the known values of r a , r b and r c :
r = ( 3 6 7 3 − 1 4 5 ) ( 2 5 6 6 − 8 2 9 ) + ( 3 6 7 3 − 1 4 5 ) ( 1 8 − 8 5 ) + ( 2 5 6 6 − 8 2 9 ) ( 1 8 − 8 5 )
And simplifying a lot, we obtain that r = 2 .
Next, from the three first equation that we obtained, we're going to solve for the three sines of △ A B C :
3 6 7 3 − 1 4 5 = 2 ( 1 + sin ( 2 A ) 1 − sin ( 2 A ) ) ⇒ sin ( 2 A ) = 1 4 5 1 4 5
2 5 6 6 − 8 2 9 = 2 ( 1 + sin ( 2 B ) 1 − sin ( 2 B ) ) ⇒ sin ( 2 B ) = 2 9 2 2 9
1 8 − 8 5 = 2 ( 1 + sin ( 2 C ) 1 − sin ( 2 C ) ) ⇒ sin ( 2 C ) = 5 2 5
Obtain the sines with the double angle formula and the Pythagoeran identity:
sin A = 1 4 5 2 4 , sin B = 2 9 2 0 and sin C = 5 4
With the inradius and the sines of the three angles of △ A B C , it's easy to obtain every side.
We know that [ A B C ] = 2 a b sin C = 2 a c sin B = 2 b c sin A and r = a + b + c 2 [ A B C ] , so form a equation system:
2 = a + b + c a b ∗ 5 4 ⇒ 2 ( a + b + c ) = 5 4 a b
2 = a + b + c a c ∗ 2 9 2 0 ⇒ 2 ( a + b + c ) = 2 9 2 0 a c
2 = a + b + c b c ∗ 1 4 5 2 4 ⇒ 2 ( a + b + c ) = 1 4 5 2 4 b c
From the first and the second equation, we get b = 2 9 2 5 c ; and from the first and the third equation, we get a = 2 9 6 c . Substitute them in any equation, let's choose the first one:
2 ( 2 9 6 c + 2 9 2 5 c + c ) = 5 4 ( 2 9 6 c ) ( 2 9 2 5 c ) ⇒ c = 2 9
With c , obtain a and b :
a = 2 9 6 ∗ 2 9 ⇒ a = 6
b = 2 9 2 5 ∗ 2 9 ⇒ b = 2 5
Finally, 1 0 0 0 0 a + 1 0 0 b + c = 6 2 5 2 9 .