The Problem Says it All!

Geometry Level 5

T ( r , θ ) = sin ( r θ ) k = 0 r 1 sin ( θ + k π r ) \large T(r,\theta)= \frac{\sin(r \theta)}{ \displaystyle \prod_{k=0}^{r-1} \sin \left (\theta +\frac{k\pi }{r} \right )}

Let S ( n , θ ) = r = 1 n T ( r , θ ) \displaystyle S(n,\theta)=\sum_{r=1}^{n}T(r,\theta) . Find the value of S ( 6 , 1 ) + T ( 7 , 2 ) S(6,1) + T(7,2) .


The answer is 127.

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2 solutions

Shashwat Shukla
Feb 4, 2015

The problem is essentially based on the fact that sin r θ = 2 r 1 k = 0 r 1 sin ( θ + k π r ) \sin {r \theta } = 2^{r-1} \prod_{k=0}^{r-1} \sin {(\theta + \frac {k\pi}{r})}

This is turn can be proven as follows:

Consider the n t h n^{th} roots of the complex number e 2 n i θ e^{2ni\theta} .

Let these roots be z 0 , z 2 . . . z n 1 z_{0},z_{2}...z_{n-1}

It's fairly obvious that z r z_{r} can be expressed as: z r = e i ( 2 θ + 2 π r n ) z_{r}=e^{i(2\theta +\frac{2\pi r}{n})}

Now, using the fact that these are the roots of the equation z n = e 2 n i θ z^n=e^{2ni\theta} , we can write:

z n e 2 n i θ = ( z z 0 ) ( z z 1 ) ( z z 2 ) . . . ( z z n 1 ) z^n-e^{2ni\theta}=(z-z_{0})(z-z_{1})(z-z_{2})...(z-z_{n-1})

Note that this equation is true for any value of z z . For the purpose of this question, make the substitution, z = 1 z=1 to get: 1 e 2 n i θ = ( 1 z 0 ) ( 1 z 1 ) ( 1 z 2 ) . . . ( 1 z n 1 ) 1-e^{2ni\theta}=(1-z_{0})(1-z_{1})(1-z_{2})...(1-z_{n-1})

Take the modulus of both sides. Consider the general term: 1 e i ( 2 θ + 2 π r n ) = e i ( θ + π r n ) e i ( θ + π r n ) e i ( θ + π r n ) = e i ( θ + π r n ) e i ( θ + π r n ) = 2 s i n ( θ + r π n ) |1- e^{i(2\theta +\frac{2\pi r}{n})}|=|e^{i(\theta +\frac{\pi r}{n})}||e^{-i(\theta +\frac{\pi r}{n})}-e^{i(\theta +\frac{\pi r}{n})}|\\=|e^{-i(\theta +\frac{\pi r}{n})}-e^{i(\theta +\frac{\pi r}{n})}|\\=|2sin(\theta+\frac{r\pi}{n})|

Do this for every term on the LHS and RHS and the required result is obtained.

Akshay Bodhare
Feb 2, 2015

In general,

sin r θ = 2 r 1 k = 0 r 1 sin ( θ + k π r ) \sin {r \theta } = 2^{r-1} \prod_{k=0}^{r-1} \sin {(\theta + \frac {k\pi}{r})}

After this the problem is very easy.

Therefore, T ( r , θ ) = 2 r 1 T(r,\theta)=2^{r-1}

S ( n , θ ) = 2 r 1 S(n,\theta)=2^r-1

Therefore,

S(6)=63

T(7)=64

Thus,Answer=127.

Note:What is interesting is that the answer is independent of θ \theta

EDIT:I don't know the proof to the identity.I started from r=1,2,3,4 and found it while searching the web for general formula of sin r θ \sin{r\theta}

@Akshay Bodhare Good work :) . Proving it isn't too hard...Let θ \theta tend to zero...Look at the relation that you get...It becomes more obvious how one can proceed...If you can prove that, the same logic will work for the general θ \theta . Will post the complete solution if no else bothers. Cheers :)

Shashwat Shukla - 6 years, 4 months ago

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Try using \ddot\smile for a ¨ \ddot\smile !!!

A Former Brilliant Member - 6 years, 4 months ago

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Thanks ¨ \ddot\smile . Oh and did you prove this result? If so, any plans to post a solution?(asking because I'd like someone else to do the honors).

Shashwat Shukla - 6 years, 4 months ago

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@Shashwat Shukla You have been posting a lot of good questions lately and pls keep on doing so. I followed you to get all the good question that you post , cheers ¨ \ddot\smile .

I don't think that I want to post a FULL solution written in LaTEX but yeah, I did cheat by searching on Google for formulae on s i n r θ sin r \theta and little help from Wolfram Alpha , to yield

T ( r , θ ) = sin ( r θ ) k = 0 r 1 sin ( θ + k π r ) = 2 r 1 \begin{aligned} T(r,\theta) = \frac{\sin (r \theta)}{\prod_{k = 0}^{r - 1} \sin (\theta + \frac{k \pi}{r})} \\ = 2^{r - 1} \end{aligned} and solving to get the answer .

BTW why is the question flagged ?

A Former Brilliant Member - 6 years, 4 months ago

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@A Former Brilliant Member Thanks again...I've been trying out your problems as well and they are really nice :) (not sure if the other smiley is worth the typing :P)...The question was flagged because I did not consider the cases where T ( r , θ ) T(r,\theta) was not defined (because s i n ( π ) sin(\pi) would come in the denominator)...But, it's been sorted out now thanks to Calvin Lin...Looking forward to seeing more awesome questions from you. Cheers.

Shashwat Shukla - 6 years, 4 months ago

@A Former Brilliant Member And also what are your views on this question ?

A Former Brilliant Member - 6 years, 4 months ago

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@A Former Brilliant Member Hmm...Number theory isn't exactly a forte of mine :P...This might take a while... But is there anything in particular that you'd like me to look out for?

Shashwat Shukla - 6 years, 4 months ago

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@Shashwat Shukla Nah, not really. It's just that reading the title of the question has had me fired up !!! But really I haven't been having any success at it plus the fact that it's only a Level 4 question and I am not able to solve it .

A Former Brilliant Member - 6 years, 4 months ago

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@A Former Brilliant Member It just became Level 5...And yea, the title is rather tantalizing...And btw, if you'd like to try something that looks very mundane on first sight but isn't, do check this out. It's Level 4 as of now.

Shashwat Shukla - 6 years, 4 months ago

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@Shashwat Shukla I've got to say this, DE is really not my forte(neither is any Calculus topic) but I'll try this and make sure that I get to the answer before midnight !!!

A Former Brilliant Member - 6 years, 4 months ago

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@A Former Brilliant Member Hey this is a Cauchy Euler equation, I think that I've nailed it(may be!!!) .

A Former Brilliant Member - 6 years, 4 months ago

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@A Former Brilliant Member So you did know about that family of equations! And you say you aren't good with DEs! And yep...That's all there is to it...So where did you first come across that particular class of DEs? Perhaps while studying electrostatics/image charges?

Shashwat Shukla - 6 years, 4 months ago

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@Shashwat Shukla Done!!!! .It seems you have already posted a solution on it .

Well, I'm not too good at calculus . It's just that some days before Jake Lai had recommended me a reading material on Discrete Calculus so after reading it and then I did a bit of reasearch of my own so I got this and this along with this . All are good for reading , I suggest you read those in your free time .

A Former Brilliant Member - 6 years, 4 months ago

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@A Former Brilliant Member Thanks a bunch! I've quite enjoyed this correspondence of ours :)...I'll try reading it in my 'free' time. Thanks again.

Shashwat Shukla - 6 years, 4 months ago

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@Shashwat Shukla Yeah, any time .

A Former Brilliant Member - 6 years, 4 months ago

I am currently working on proof.Is it by any chance related to complex number and nth root of unity.

Akshay Bodhare - 6 years, 4 months ago

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Nailed it right on the head! Great job :)

Shashwat Shukla - 6 years, 4 months ago

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@Shashwat Shukla I thought up the idea while trying to solve some calculus problem.Btw the internet does not have any proof for this.

Akshay Bodhare - 6 years, 4 months ago

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@Akshay Bodhare I made up this result when I tried generalising the θ = 0 \theta=0 case... And whoah, no proof online...Now that's surprising to say the least...Yours might then become the first :D (if you choose to publish that is)

Shashwat Shukla - 6 years, 4 months ago

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@Shashwat Shukla Does Euler DE come in JEE?

Akshay Bodhare - 6 years, 4 months ago

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@Akshay Bodhare Nope. Haven't seen any question in the archive at all.

Shashwat Shukla - 6 years, 4 months ago

@Akshay Bodhare But isn't it quite easy Akshay .

A Former Brilliant Member - 6 years, 4 months ago

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@A Former Brilliant Member Easy but time consuming and tedious.I always go for questions in which we can get the answer with the least bit of effort.

Akshay Bodhare - 6 years, 4 months ago

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@Akshay Bodhare Yeah, right .

A Former Brilliant Member - 6 years, 4 months ago

@Shashwat Shukla I am not getting the answer.Please publish you proof.

Akshay Bodhare - 6 years, 4 months ago

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@Akshay Bodhare Okay, will do tomorrow. Do you check back again later. But if you want to work it out yourself, consider the nth roots of e i θ e^{i\theta} . Writing down the polynomial whose roots are these n roots and then taking the modulus of both sides with a little manipulation yields the result. But I'll post the full thing ASAP. Cheers.

Shashwat Shukla - 6 years, 4 months ago

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@Shashwat Shukla Got it x r = e i θ x^r=e^{i\theta} and then the product of all the roots is the constant term.Thanks.It seems very easy now but was so hard a moment ago.

Akshay Bodhare - 6 years, 4 months ago

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@Akshay Bodhare You're welcome :)

Shashwat Shukla - 6 years, 4 months ago

@Shashwat Shukla @Shashwat Shukla , @Akshay Bodhare both of you try this question later. an easy 310 point question . Goodnight to both of you !!

A Former Brilliant Member - 6 years, 4 months ago

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@A Former Brilliant Member Goodnight and thanks :)

Shashwat Shukla - 6 years, 4 months ago

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@Shashwat Shukla Good night.

Akshay Bodhare - 6 years, 4 months ago

Akshay the identity can be proved by the separation of the product and application of the transformation of product of sines to sum of cosines then we find the numerator to be if the form sin(rtheta) and then the sum becomes very basic like you pointed out

Anirudh Chandramouli - 5 years, 11 months ago

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