Consider Δ A B C such that A B = 1 3 , B C = 1 4 , A C = 1 5
Let A D ⊥ B C , B E ⊥ A C , C F ⊥ A B and let H be its orthocenter
Let R ( A B C ) denote circumradius of Δ A B C
Let r 0 be inradius and r 1 , r 2 , r 3 be the exradii of Δ A B C
Then find the value of
R ( A B C ) + R ( H A B ) + R ( H B C ) + R ( H A C ) + i = 0 ∑ 3 r i
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Guess I lost ! But you've got to agree that my question was easier .
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Yup! xD :)
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@A Former Brilliant Member – I will wait for you :)
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In Δ A B C , R ( Δ A B C ) = S i n A 1 4 = S i n B 1 5 = S i n C 1 3
In Δ H B C , R ( Δ H B C ) = S i n ( B + C ) 1 4
But we know A + B + C = π
Hence
R ( Δ H B C ) = S i n ( π − A ) 1 4 = S i n A 1 4 = R ( Δ A B C )
Similarily ,
R ( Δ A B C ) = R ( Δ H A C ) = R ( Δ H B A ) = R ( Δ H B C )
a r ( Δ A B C ) = 8 4 ⇒ R ( Δ A B C ) = 4 a r ( Δ A B C ) a b c
r 0 = s a r ( Δ A B C )
r 1 + r 2 + r 3 = r 0 + 4 R
Hence we have to find 8 R + 2 r = 4 × 8 4 8 × 1 3 × 1 4 × 1 5 + 2 1 2 × 8 4 = 7 3
Note: s is semiperimeter.
Nice one. I have to yet learn trigo ...... !
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Let P , Q , R be the reflections of H in BC , AC , AB respectively.
m ∠ B A C = a , m ∠ A B C = b , m ∠ A C B = c a ∣ = 9 0 − a , b ∣ = 9 0 − b , c ∣ = 9 0 − c
By angle chasing we can clearly get that quadrilaterals ABPC , ABCQ , ARBC are cyclic since their opposite angles are supplementary.
⟹ R ( A B C ) = R ( B P C ) = R ( A Q C ) = R ( A R B )
Since Δ B H C ≅ Δ B P C Δ A H C ≅ Δ A Q C Δ A H B ≅ Δ A R B ⟹ R ( A B C ) = R ( H A B ) = R ( H B C ) = R ( H A C )
Area of Δ A B C = 8 4 (Using Heron's formula).Also ,
Δ A B C = 4 R product of all sides ⇒ 4 R = 8 4 2 7 3 0 = 3 2 . 5 where 'R' is circumradius of Δ A B C
Area of Δ A B C = r 0 . s ⇒ r 0 = 2 1 8 4 = 4 where 's' is the semiperimeter of the triangle.
We also have the relation r 1 + r 2 + r 3 = r 0 + 4 R
i = 0 ∑ 3 = 4 R + 2 r 0 = 3 2 . 5 + 8 = 4 0 . 5
So , R ( A B C ) + R ( H A B ) + R ( H B C ) + R ( H A C ) + i = 0 ∑ 3 r i
⇒ 4 R + 4 R + 2 r 0 = 3 2 . 5 + 4 0 . 5 = 7 3
@Azhaghu Roopesh M