The area of △ A B C is 40. Its incircle intersects C B at E . ∠ C A B is 7 5 ∘ . What is C E ⋅ E B ?
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.
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For the sake of completeness, the closed-form of C E ⋅ E B is 4 0 ( − 2 − 2 + 3 + 6 )
Can I suggest reducing the number of digits needed, I had to search for an online calculator which could do the 11.
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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 ) denote the least degree monic polynomial where the numerical value of C E ⋅ E B is one of its roots. What is f ( 1 ) ?
And the answer is 1 4 + 3 2 0 ⋅ 1 3 + 3 2 0 0 ⋅ 1 2 − 5 1 2 0 0 0 ⋅ 1 + 2 5 6 0 0 0 = 2 0 5 1 5 2 1 .
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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.
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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! :)
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Let D be the point of tangency on A B , F be the point of tangency on A C , and O be the center of the incircle, and let a = C F = C E , b = B E = B D , and c = A F = A D .
Then the area of the triangle is given by 4 0 = 2 1 ( a + c ) ( b + c ) sin 7 5 ° , which rearranges to a b + a c + b c + c 2 = 8 0 csc 7 5 ° .
From △ O A D , the inradius is r = c tan 3 7 . 5 ° , but also r = s △ A B C A △ A B C = a + b + c 4 0 , so c tan 3 7 . 5 ° = a + b + c 4 0 , which rearranges to a c + b c + c 2 = 4 0 cot 3 7 . 5 ° .
Combining a b + a c + b c + c 2 = 8 0 csc 7 5 ° and a c + b c + c 2 = 4 0 cot 3 7 . 5 ° gives C E ⋅ E B = a b = 8 0 csc 7 5 ° − 4 0 cot 3 7 . 5 ° ≈ 3 0 . 6 9 3 0 7 9 5 1 9 1 5 8 4 , making the sum of the first 1 1 digits to the right of the decimal point 6 + 9 + 3 + 0 + 7 + 9 + 5 + 1 + 9 + 1 + 5 = 5 5 .