Mangle Angle Triangle

Geometry Level 5

The area of A B C \triangle ABC is 40. \hspace{1mm} Its incircle intersects C B CB at E E . C A B \hspace{1mm} \angle CAB is 7 5 75^\circ . \hspace{1mm} What is C E E B CE\cdot EB ?

Find a closed-form solution, convert it to decimal, and submit the sum of the first 11 digits to the right of the decimal point.


The answer is 55.

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1 solution

David Vreken
Mar 1, 2021

Let D D be the point of tangency on A B AB , F F be the point of tangency on A C AC , and O O be the center of the incircle, and let a = C F = C E a = CF = CE , b = B E = B D b = BE = BD , and c = A F = A D c = AF = AD .

Then the area of the triangle is given by 40 = 1 2 ( a + c ) ( b + c ) sin 75 ° 40 = \frac{1}{2}(a + c)(b + c) \sin 75° , which rearranges to a b + a c + b c + c 2 = 80 csc 75 ° ab + ac + bc + c^2 = 80 \csc 75° .

From O A D \triangle OAD , the inradius is r = c tan 37.5 ° r = c \tan 37.5° , but also r = A A B C s A B C = 40 a + b + c r = \cfrac{A_{\triangle ABC}}{s_{\triangle ABC}} = \cfrac{40}{a + b + c} , so c tan 37.5 ° = 40 a + b + c c \tan 37.5° = \cfrac{40}{a + b + c} , which rearranges to a c + b c + c 2 = 40 cot 37.5 ° ac + bc + c^2 = 40 \cot 37.5° .

Combining a b + a c + b c + c 2 = 80 csc 75 ° ab + ac + bc + c^2 = 80 \csc 75° and a c + b c + c 2 = 40 cot 37.5 ° ac + bc + c^2 = 40 \cot 37.5° gives C E E B = a b = 80 csc 75 ° 40 cot 37.5 ° 30.6930795191584 CE \cdot EB = ab = 80 \csc 75° - 40 \cot 37.5° \approx 30.6930795191584 , making the sum of the first 11 11 digits to the right of the decimal point 6 + 9 + 3 + 0 + 7 + 9 + 5 + 1 + 9 + 1 + 5 = 55 6 + 9 + 3 + 0 + 7 + 9 + 5 + 1 + 9 + 1 + 5 = \boxed{55} .

For the sake of completeness, the closed-form of C E E B CE \cdot EB is 40 ( 2 2 + 3 + 6 ) 40 (-2 - \sqrt2 + \sqrt3 + \sqrt6)

Pi Han Goh - 3 months, 1 week ago

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Thank you very much!

David Vreken - 3 months, 1 week ago

Can I suggest reducing the number of digits needed, I had to search for an online calculator which could do the 11.

Razzi Masroor - 3 months ago

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I have to agree with this. @Fletcher Mattox

A simple remedy for this is to just ask for the value to 5 significant figures. 11 seems like a huge overkill.


On the other hand, if I were the problem setter, I would have phrased it as

Let f ( x ) f(x) denote the least degree monic polynomial where the numerical value of C E E B CE\cdot EB is one of its roots. What is f ( 1 ) f(1) ?


And the answer is 1 4 + 320 1 3 + 3200 1 2 512000 1 + 256000 = 2051521 . 1^4 + 320 \cdot 1^3 + 3200\cdot1^2 - 512000 \cdot1 + 256000 = \boxed{2051521}.

Pi Han Goh - 3 months ago

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Thank you! This is an excellent idea. I have been thinking along similar lines. The solution to a problem I am composing now is the root of a quartic equation. I was thinking of asking the solver to find the minimal polynomial and submit, say, the sum of its coefficients.

Fletcher Mattox - 3 months ago

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@Fletcher Mattox And if you think 11 digits is excessive, wait till you see the problem where I asked for 50 digits! :)

Fletcher Mattox - 3 months ago

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