Sin-o-metry

Geometry Level 4

r = 1 12 sin ( r x ) = 0 \large \prod_{r = 1}^{12} \sin (rx) = 0

What is the number of solutions of x x satisfying the equation above in the interval ( 0 , π ] ? (0,\pi]?

12 18 22 36 42 46

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1 solution

Arian Tashakkor
May 21, 2015

There should definitely be an easier answer but my approach was analyzing all the possible solutions.As hard as it may seem, it took me only 3-4 mins to find them all out using the facts below:

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  1. First off , note that s i n x = 0 sinx = 0 has only one answer in the given interval and that is x = π x=\pi ,keeping that in mind ,for the next equations we don't have to count the case of x = π x = \pi as in s i n 2 x = 0 x = 2 π 2 = π sin2x=0 \rightarrow x=\frac {2\pi} {2} = \pi .

  2. Start solving the equations one by one from 1 to 12 not in any other order.How does that help?Well you can see that if - for example - you are analyzing the case of s i n 12 x = 0 sin12x = 0 you just need to write down all the fractions from 11 12 1 12 \frac{11}{12} \dots \frac{1}{12} but whenever a fraction is reducible be quite sure that , that case has already been counted.For example not long after starting your analysis you'll be analyzing 10 12 \frac {10}{12} which can be reduced to 5 6 \frac{5}{6} and this case is already analyzed in s i n 6 x = 0 sin6x=0

\quad

I don't know if I was able to convey my meaning to you well enough or not but if there was any problem understanding this just comment below my solution.I'll answer whenever possible.

Moderator note:

Yes, this is the standard approach, although you working could be improved on. There's a shortcut to this problem, can you find it? Hints:

  1. Try for small values first: replace 12 12 with a smaller number, say 5 5 .

  2. State all the values of x x in its lowest form, which one is irreducible?

  3. How many are irreducible for sin ( n x ) \sin(nx) ?

  4. Can you spot a pattern?

It is easier to count the fractions using the Euler phi function: http://primes.utm.edu/glossary/xpage/eulersphi.html Note that the answer is then ϕ ( 1 ) + ϕ ( 2 ) + + ϕ ( 12 ) = 46 \phi(1) + \phi(2) + \ldots + \phi(12) = \boxed{46}

Harrison Wang - 6 years ago

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Ahhh!Now I see it!Euler phi function exactly does it for me since it counts the the numbers that would make the fraction irreducible (since the numbers in the fraction have to be relatively prime )

Arian Tashakkor - 6 years ago

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