Triangles (1).

Geometry Level 5

The lengths of tangents drawn from A , B , A, B, and C C to the incircle ( Σ ) (\Sigma) of A B C \triangle ABC are 8 , 6 , 8,6, and 3 , 3, respectively. The lengths of the parts of the tangents which are drawn parallel to sides B C , C A , BC, CA, and A B AB and intercepted between the sides of the triangle are p , q , p,q, and r , r, respectively.

Find the value of p + q + r p q r . \left| \dfrac{p+q+r}{p-q-r} \right|.

This figure is for representative purposes only. This figure is for representative purposes only.


The answer is 5.

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2 solutions

Let the side of the triangles be represented by a, b & c

Therefore length of tangent from vertex A is equal to s-a and similarly s-b and s-c from vertices B & C respectively

Given s-a = 8, s-b = 6, s-c = 3 Adding the three we get

3s-(a+b+c) = 17 therefore s = 17

thus a = 9, b = 11, c = 14

Also length of tangent from vertex A = semi-perimeter of triangle ADE

i.e. 8 = c . p a + b . p a + p 2 \frac { \frac { { c }{ .p } }{ a } +\frac { { b }.{ p } }{ a } +{ p } }{ 2 }

which gives p = 16 a a + b + c \frac { { 16 }{ a } }{ { a }+{ b }+{ c } }

therefore q = 12 b a + b + c \frac { { 12 }{ b } }{ { a }+{ b }+{ c } } r = 6 c a + b + c \frac { { 6 }{ c } }{ { a }+{ b }+{ c } }

thus p + q + r p q r \left| \frac { { p }+{ q }+{ r } }{ { p }-{ q }-{ r } } \right| = 360 72 \left| \frac { 360 }{ -72 } \right| = 5

Good solution bro. But your approach is too straightforward. Such as how one comes from here

Also length of tangent from vertex A = semi-perimeter of triangle ADE

to here

i.e. 8 = c a p + b a p + p 2 8 = \dfrac{ \frac{c}{a} \cdot p + \frac{b}{a} \cdot p + p}{2}

needs to be explained via similarity of triangles. I understand that you're constricted to the point due to less tools. Here are some things i can help you with for future solutions:

Tapas Mazumdar - 3 years, 2 months ago

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Too long to write

A Former Brilliant Member - 3 years, 2 months ago

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Well thank u for providing me with these tools

A Former Brilliant Member - 3 years, 2 months ago

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@A Former Brilliant Member You're welcome. :)

Tapas Mazumdar - 3 years, 2 months ago

c = 8 + 6 = 14 , . . . . . . . . . . . . . . a = 6 + 3 = 9 , . . . . . . . . . . . . b = 8 + 3 = 11 , s = 1 / 2 ( a + b + c ) = 17 A r e a = s ( s a ) ( s b ) ( s c ) = 49.4793. i n r a d i u s , r = 2 A r e a s = 2.91043. A l t i t u d e s h a = 2 a r e a a = 2 49.4793 9 = 10.9954. A l t i t u d e s h b = 2 a r e a b = 2 49.4793 11 = 8.99624. A l t i t u d e s h c = 2 a r e a c = 2 49.4793 14 = 7.06847. p = a ( h a 2 r ) h a = 9 ( 10.9954 5.82086 ) 10.9954 = 4.23545. q = a ( h b 2 r ) h b = 9 ( 8.99624 5.82086 ) 8.99624 = 3.88259. r = a ( h c 2 r ) h c = 9 ( 7.06847 5.82086 ) 7.06847 = 2.47097. S o p + q + r p q r = 5. c=8+6=14,..............a=6+3=9,............b=8+3=11,\\ s=1/2*(a+b+c)=17 \\ Area~=\sqrt {s*(s-a)*(s-b)*(s-c) }=49.4793.\\ in-radius,r=\dfrac{2*Area} s =2.91043.\\ Altitudes~h_a=\dfrac{2*area} a=\dfrac{ 2*49.4793} 9=10.9954.\\ Altitudes~h_b=\dfrac{2*area} b=\dfrac{ 2*49.4793}{ 11}=8.99624.\\ Altitudes~h_c=\dfrac{2*area} c=\dfrac{ 2*49.4793} {14}=7.06847.\\ \therefore~p=\dfrac{a*(h_a-2r)}{h_a} =\dfrac{9*(10.9954-5.82086)}{10.9954} = 4.23545.\\ \therefore~q=\dfrac{a*(h_b-2r)}{h_b} =\dfrac{9*(8.99624-5.82086)}{8.99624} = 3.88259.\\ \therefore~r=\dfrac{a*(h_c-2r)}{h_c} =\dfrac{9*(7.06847-5.82086)}{7.06847} = 2.47097.\\ So~~~\dfrac{p+q+r}{p-q-r}=\Huge~~~\color{#D61F06}{5}.

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