400 followers problem!

Geometry Level 4

Consider Δ A B C \Delta ABC such that A B = 13 , B C = 14 , A C = 15 AB = 13, BC = 14, AC = 15

Let A D B C , B E A C , C F A B AD \perp BC, BE \perp AC, CF \perp AB and let H H be its orthocenter

Let R ( A B C ) R(ABC) denote circumradius of Δ A B C \Delta ABC

Let r 0 r_0 be inradius and r 1 , r 2 , r 3 r_1, r_2, r_3 be the exradii of Δ A B C \Delta ABC

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 R(ABC) + R(HAB) + R(HBC) + R(HAC) + \sum_{i=0}^3 r_i


The answer is 73.

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3 solutions

Nihar Mahajan
Mar 25, 2015

Let P , Q , R be the reflections of H in BC , AC , AB respectively. \text{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 = 90 a , b = 90 b , c = 90 c m\angle BAC = a \quad, \quad m\angle ABC = b \quad , \quad m\angle ACB = c \\ a^| = 90-a \quad,\quad b^| = 90-b\quad,\quad c^| = 90-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 ) \implies R(ABC)=R(BPC)=R(AQC)=R(ARB)

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 ) \Delta BHC \cong \Delta BPC \quad\Delta AHC\cong \Delta AQC\quad\Delta AHB \cong \Delta ARB \\ \implies \boxed{R(ABC)=R(HAB)=R(HBC)=R(HAC)}

Area of Δ A B C = 84 \Delta ABC = 84 (Using Heron's formula).Also ,

Δ A B C = product of all sides 4 R 4 R = 2730 84 = 32.5 \Delta ABC = \dfrac{\text{product of all sides}}{4R} \Rightarrow 4R = \dfrac{2730}{84} = 32.5 where 'R' is circumradius of Δ A B C \Delta ABC

Area of Δ A B C = r 0 . s r 0 = 84 21 = 4 \Delta ABC = r_0.s \Rightarrow r_0 = \dfrac{84}{21} = 4 where 's' is the semiperimeter of the triangle.

We also have the relation r 1 + r 2 + r 3 = r 0 + 4 R r_1+r_2+r_3=r_0+4R

i = 0 3 = 4 R + 2 r 0 = 32.5 + 8 = 40.5 \displaystyle \sum_{i=0}^3 = 4R +2r_0= 32.5 + 8 = 40.5

So , R ( A B C ) + R ( H A B ) + R ( H B C ) + R ( H A C ) + i = 0 3 r i \large R(ABC)+R(HAB)+R(HBC)+R(HAC)+\displaystyle \sum_{i=0}^3 r_i

4 R + 4 R + 2 r 0 = 32.5 + 40.5 = 73 \Rightarrow 4R + 4R + 2r_0 = 32.5+40.5 = \huge\boxed{73}

@Azhaghu Roopesh M

Guess I lost ! But you've got to agree that my question was easier .

@Nihar Mahajan

A Former Brilliant Member - 6 years, 2 months ago

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Yup! xD :)

Nihar Mahajan - 6 years, 2 months ago

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Well , I'll be back with a better question and then I'll challenge you :P

A Former Brilliant Member - 6 years, 2 months ago

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@A Former Brilliant Member Your 400 followers and my 500 followers problem ... :)

Nihar Mahajan - 6 years, 2 months ago

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@Nihar Mahajan That's not possible since your followers grow exponentially and you know that ;)

A Former Brilliant Member - 6 years, 2 months ago

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@A Former Brilliant Member I will wait for you :)

Nihar Mahajan - 6 years, 2 months ago

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@Nihar Mahajan Haha sure .

A Former Brilliant Member - 6 years, 2 months ago

@Nihar Mahajan If I , for e.g get 400 followers , you'll be at 600-650 !

A Former Brilliant Member - 6 years, 2 months ago
Gautam Sharma
Mar 26, 2015

In Δ A B C \Delta ABC , R ( Δ A B C ) = 14 S i n A = 15 S i n B = 13 S i n C R(\Delta ABC)=\frac {14}{SinA}=\frac {15}{SinB}=\frac {13}{SinC}

In Δ H B C \Delta HBC , R ( Δ H B C ) = 14 S i n ( B + C ) R(\Delta HBC)=\frac {14}{Sin(B+C)}

But we know A + B + C = π A+B+C=\pi

Hence

R ( Δ H B C ) = 14 S i n ( π A ) = 14 S i n A R(\Delta HBC)=\frac {14}{Sin(\pi-A)}=\frac {14}{SinA} = R ( Δ A B C ) R(\Delta ABC)

Similarily ,

R ( Δ A B C ) R(\Delta ABC) = R ( Δ H A C ) R(\Delta HAC) = R ( Δ H B A ) R(\Delta HBA) = R ( Δ H B C ) R(\Delta HBC)

a r ( Δ A B C ) = 84 R ( Δ A B C ) = a b c 4 a r ( Δ A B C ) ar(\Delta ABC) =84 \Rightarrow R(\Delta ABC)=\frac {abc}{4ar(\Delta ABC)}

r 0 = a r ( Δ A B C ) s r_0=\frac{ar(\Delta ABC)}{s}

r 1 + r 2 + r 3 = r 0 + 4 R r_1+r_2+r_3=r_0+4R

Hence we have to find 8 R + 2 r 8R+2r = 8 × 13 × 14 × 15 4 × 84 \frac{8\times 13\times 14\times 15}{4\times 84} + 2 × 84 21 = 73 \frac{2\times 84}{21} = \boxed{73}

Note: s is semiperimeter.

Nice one. I have to yet learn trigo ...... !

Nihar Mahajan - 6 years, 2 months ago

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I can very well understand it now :)

Nihar Mahajan - 5 years, 10 months ago
Aakash Khandelwal
Mar 30, 2015

did by coordinates

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