Approximate the answer based on distances between SSS triangles' upper vertices.

Geometry Level 3

Compute d d d-\lceil d \rfloor where \lceil \cdot \rfloor means to round to nearest integer. In this case, the value is distinctly close to an integer so that no disambiguation rules are needed.


The answer is 8.57367E-8.

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3 solutions

Pi Han Goh
Oct 9, 2020

Let us label the angles as such. Using cosine rule , we have

cos ( θ 1 ) = d 2 + 2 3 2 2 7 2 2 d 23 , cos ( θ 2 ) = d 2 + 1 6 2 2 2 2 2 d 16 . \cos(\theta_1) = \dfrac{d^2 + 23^2 - 27^2}{2\cdot d \cdot 23}, \quad \cos(\theta_2) = \dfrac{d^2 + 16^2-22^2}{2\cdot d \cdot 16}.

With compound angle formula , cos ( A + B ) = cos ( A ) cos ( B ) sin ( A ) sin ( B ) \cos(A+B) = \cos(A) \cos(B) - \sin(A) \sin(B) , we obtain

cos ( 36 0 ( θ 1 + θ 2 ) ) = cos ( θ 1 + θ 2 ) = cos ( θ 1 ) cos ( θ 2 ) sin ( θ 1 ) sin ( θ 2 ) = 2 3 2 + 1 6 2 3 0 2 2 23 16 = 5 32 . \cos(360^\circ - (\theta_1 + \theta_2)) = \cos(\theta_1 + \theta_2) = \cos(\theta_1) \cos(\theta_2) - \sin(\theta_1) \sin(\theta_2) = \dfrac{23^2 + 16^2 - 30^2}{2\cdot23\cdot16} = -\frac5{32}.

Because sin ( A ) = 1 cos 2 ( A ) \sin(A) = \sqrt{1-\cos^2(A)} for 0 < A < 18 0 0^\circ < A < 180^\circ , the equation above can be expanded and simplified to:

900 d 4 921 315 d 2 + 42 983 536 = 0. 900 d^4 - \num{921315} d^2 + \num{42983536}= 0 . Using quadratic formula , and knowing that d > 0 d>0 is super duper close to an integer, d = 1 2 61 421 30 23 2 388 759 15 7 + 0.00000008573675 . d = \frac12\sqrt{\dfrac{\num{61421}}{30} - \dfrac{23}2 \sqrt{\dfrac{\num{388759}}{15} }} \approx 7 + \boxed{0.00000008573675}.


Also note that the inradius of the circumcircle of the entire triangle is s ( s a ) ( s b ) ( s c ) s \dfrac{ \sqrt{s(s-a)(s-b)(s-c)}}s , where a , b , c = 27 , 22 , 30 , s = a + b + c 2 a,b,c = 27,22,30, s = \frac{a+b+c}2 , then r = 16 625 316 < 8 r = \sqrt{ \frac{\num{16625}}{316}} < 8 . So d < 2 r < 16 d < 2r < 16 ( d d is shorter than the diameter of the circumcircle). Thus, even if we're not told that d d is super duper close to an integer, then the other positive root of d d (in the quartic equation above) is an extraneous root.

On the other hand, by trial and error, 900 X 4 921 315 X 2 + 42 983 535 = 0 900 X^4 - \num{921315} X^2 + \num{42983535 } = 0 has a root 7, so 900 X 4 921 315 X 2 + 42 983 535 + 1 = 0 900 X^4 - \num{921315} X^2 + \num{42983535}+ 1=0 has a root larger than 7, but only eensy larger than 7.

The issue is the needed precision of the computations to be able to see the answer. The subtraction of two nearly equal floating point (scientific notation number to some people) causes significant loss of precison. In this case, if the computations were done on a slide rule (at best, four digits of precison) or typical hand calculators (eight digits of precison) are not sufficient to see the difference clearly and with sufficient precison for Brilliant's floating point answer handling (about three percent or between two and three digits of precison). Twelve digits or more are sufficient. Most calculator programs on smart cellphones and Wolfram|Alpha have sufficient precison.

Callie Ferguson
Oct 8, 2020

I solved this problem using GeoGebra - check out the link below to see what I did.

https://www.geogebra.org/calculator/tjbwrbrg

Oh, also, to the person who wrote this problem: Maybe include in the description that you need to use scientific notation to solve the problem! I typed 0 in and got it wrong, and I was so sure of myself. But I figured it out anyway haha

Callie Ferguson - 8 months ago

That is the intended lesson for this problem. The "distinctly close to an integer" was also a hint.

What was the intended lesson? And the "distinctly close to an integer" part made me think it was something like, for example, 8.92 (or something where the decimal was close to 1 or 0).

Callie Ferguson - 8 months ago

Yes. This is not a problem to be solved on a slide rule. In fact, most eight digit hand calculators would have problems with the calculations. As most hand calculators have even number of digits, a hand calculator with 12 digits or more could do the problem, e.g., TI 89, HP 4x series, and the HP Prime.

Okay, that makes sense. I had to use online graphing calculators because I lost my hand-held one. Grr!

Callie Ferguson - 8 months ago

:-) I had more problems generating the illustration than doing the problem.

Assuming that the base of both triangles is horizontal and that the left hand ends of the bases are at the origin, the coordinates of the vertices of the larger triangle are { 0 , 0 } , { 30 , 0 } , { 229 12 , 52535 12 } \{0,0\},\{30,0\},\left\{\frac{229}{12},\frac{\sqrt{52535}}{12}\right\} and the coordinates of the vertices of the smalle triangle are { 0 , 0 } , { 30 , 0 } , { 391 20 , 23 111 20 } \{0,0\},\{30,0\},\left\{\frac{391}{20},\frac{23 \sqrt{111}}{20}\right\} . The distance d d is ( 52535 12 23 111 20 ) 2 + 49 225 \sqrt{\left(\frac{\sqrt{52535}}{12}-\frac{23 \sqrt{111}}{20}\right)^2+\frac{49}{225}} . That value evaluates after rounding to 7. 7. The difference numerically between 7 7 and d d is about 8.573674836753753E-8 \text{8.573674836753753E-8} .

I think you will find that the word you want is disambiguation

Mark Hennings - 8 months ago

As it does not change my intent, I changed the spelling in the problem.

Very nice near-integer solution considering how small the side-lengths are. Did you discover this yourself?

Chris Lewis - 8 months ago

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I'm also incredibly curious how he managed to find such "nearly an integer" in an entire setup with no non-integers present. Reminds me of the Fermat's last theorem near misses .

Pi Han Goh - 8 months ago

Web searches are very helpful. The search phrase I used is "almost integer". I picked from the more extreme examples to raise the computation precision issue.

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