MHB Evaluate a floor function involving trigonometric functions

AI Thread Summary
The discussion revolves around evaluating the expression involving trigonometric functions: $$\left\lfloor{\tan^4 \frac{3\pi}{7}+\tan^4 \frac{2\pi}{7}+2\left(\tan^2 \frac{3\pi}{7}+\tan^2 \frac{2\pi}{7}\right)}\right\rfloor$$. Participants suggest that the answer is 412 and explore various trigonometric identities to simplify the problem. Key identities discussed include relationships involving cosine and tangent products, which may help in deriving the solution. A perfect square trinomial pattern is identified, leading to a potential simplification using secant functions. The conversation emphasizes the complexity of the problem and the need for deeper exploration of trigonometric properties.
anemone
Gold Member
MHB
POTW Director
Messages
3,851
Reaction score
115
Evaluate $$\left\lfloor{\tan^4 \frac{3\pi}{7}+\tan^4 \frac{2\pi}{7}+2\left(\tan^2 \frac{3\pi}{7}+\tan^2 \frac{2\pi}{7}\right)}\right\rfloor$$.

Hi MHB,

I don't know how to solve the above problem, as I have exhausted all possible methods that I could think of, and I firmly believe there got to be an easy way to crack it because this is a competition problem...any help, please?:)
 
Mathematics news on Phys.org
Re: Evaluate a floor function involves of trigonometric functions

Hmm. That is, indeed, a tough one. The answer, according to my calculator, is $412$, but how to get that? I'm thinking esoteric trig identities are the way to go. Here's one that might be useful:
$$\cos\left(\frac{\pi}{7}\right)\cos\left(\frac{2\pi}{7}\right)\cos\left(\frac{3\pi}{7}\right)=\frac18.$$
Here's another:
$$\prod_{k=1}^{m}\tan\left(\frac{k\pi}{2m+1}\right)=\sqrt{2m+1}.$$
Fleshing this out for your case yields
$$\tan\left(\frac{\pi}{7}\right)\tan\left(\frac{2\pi}{7}\right)\tan\left(\frac{3\pi}{7}\right)=\sqrt{7}.$$
We can combine these two together to get
$$\sin\left(\frac{\pi}{7}\right)\sin\left(\frac{2\pi}{7}\right)\sin\left(\frac{3\pi}{7}\right)=\frac{\sqrt{7}}{8}.$$
Hmm. Squaring some of your expressions looks like we might be able to do something here.

I also noticed that there's a perfect square trinomial pattern hidden in your original expression:
\begin{align*}
&\left\lfloor\tan^4\left(\frac{3\pi}{7}\right)+\tan^4\left(\frac{2\pi}{7}\right)+2\left(\tan^2\left(\frac{3\pi}{7}\right)+\tan^2\left(\frac{2\pi}{7}\right)\right)\right\rfloor \\
=&\left\lfloor\left(\tan^2\left(\frac{3\pi}{7}\right)+1\right)^{\!2}+\left(\tan^2\left(\frac{2\pi}{7}\right)+1\right)^{\!2}
-2\right\rfloor \\
=&\left\lfloor \sec^4\left(\frac{3\pi}{7}\right)+\sec^4\left(\frac{2\pi}{7}\right)-2\right\rfloor.\end{align*}

I'm not sure where to go from here; does this give you any ideas?
 
Thanks so much Ackbach for your reply!

I will think of it based on your observations and hopefully I can crack it soon and when I have done so, I sure will post back...it may take a while as I am very, very busy these days...
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Fermat's Last Theorem has long been one of the most famous mathematical problems, and is now one of the most famous theorems. It simply states that the equation $$ a^n+b^n=c^n $$ has no solutions with positive integers if ##n>2.## It was named after Pierre de Fermat (1607-1665). The problem itself stems from the book Arithmetica by Diophantus of Alexandria. It gained popularity because Fermat noted in his copy "Cubum autem in duos cubos, aut quadratoquadratum in duos quadratoquadratos, et...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Back
Top