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Induction to prove an inequality

  1. Dec 26, 2007 #1
    1. The problem statement, all variables and given/known data
    For all x in the interval [tex] 0 \leq x \leq \pi [/tex], prove that [tex]|\sin{nx}| \leq n\sin{x}[/tex]
    where n is a nonnegative integer.

    2. Relevant equations

    3. The attempt at a solution
    It is obviously true for n=0. Assume this result is true for n=k. Then

    [tex]|\sin(k+1)x| = |\sin kx \cos x + \sin x \cos kx | [/tex]

    and I am not sure what to do with that. I think the fact that cosine is nonnegative in this interval might be useful.
    Last edited: Dec 26, 2007
  2. jcsd
  3. Dec 26, 2007 #2


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    It seems like the next thing is to apply the triangle inequality:
    [tex]|a + b| \leq |a| + |b|[/tex]
  4. Dec 26, 2007 #3


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    And cosine isn't nonnegative over that interval, sine is. But |cos| is less than or equal to one. Both of those will be useful.
  5. Dec 26, 2007 #4
    Yeah. I got it.
  6. Jan 10, 2008 #5


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    I suppose I might have just let this be, but I thought there might be a reasonably nice non-inductive proof for this proposition. I'd been thinking about this occasionally since this was posted and was finally able now to complete the step I was missing.

    The function sin(nx) has a period 2(pi)/n and certainly equals
    (n sin x) at x = 0 and x = pi. On the interval (0, pi), (n sin x) will complete a single non-negative half-cycle with its maximum y = n at x = (pi)/2 , while |sin(nx)| will have a set of n non-negative "bumps" with unit amplitude. The first local maximum for |sin(nx)| occurs at
    x = (pi)/2n , while (n sin x) will have reached the value
    [n sin({pi}/2n)] > 1 . (In fact, in the limit as n approaches infinity, the ratio of the values of these two functions at that point approaches (pi)/2 .) So we can conclude that (n sin x) > |sin(nx)| for the interval ( {pi}/2n , {(2n-1)·pi}/2n ).

    It remains to show that this same inequality holds on (0, (pi)/2n) ; a similar argument will hold on ( {(2n-1)·pi}/2n , pi ) by symmetry. Consider the function f(x) = (n sin x) - |sin(nx)| = (n sin x) - sin(nx) on this interval. We already know f(0) = 0 and f( {pi}/2n ) > 0 , so we want to assure that there are no other zeroes for f(x) in the interval. We find

    f'(x) = (n cos x) - (n cos(nx) ) = n·( cos x - cos (nx) ) .

    Since we have integral n > 1 , we can just look at cos x - cos (nx) . But, by the sum-to-product trigonometric relations,

    cos x - cos (nx) =

    -2 · sin[ {(n+1)/2}·x ] · sin[ {(1-n)/2}·x ] =

    2 · sin[ {(n+1)/2}·x ] · sin[ {(n-1)/2}·x ] > 0 .

    So f(x) is only increasing on the interval ( 0 , (pi)/2n ), thus there are no zeroes there. Therefore, the proposition for the inequality holds everywhere on (0, pi).
    Last edited: Jan 10, 2008
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