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Math Help - Trigonometry

  1. #1
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    Trigonometry

    Why does \cos x \cos (\pi / 4) -\sin x \sin (\pi/4) equal \frac{1}{\sqrt2}(\cos x - \sin x)?

    Thanks in advance.
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  2. #2
    MHF Contributor kalagota's Avatar
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    Quote Originally Posted by Air View Post
    Why does \cos x \cos (\pi / 4) -\sin x \sin (\pi/4) equal \frac{1}{\sqrt2}(\cos x - \sin x)?

    Thanks in advance.
    \cos (\pi / 4) = \sin (\pi / 4)  = \frac{1}{\sqrt{2}}
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  3. #3
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    what is \cos \frac{\pi}{4} ? and \sin \frac{\pi}{4}

    Did you notice that this can also be written as \cos (x +\frac{\pi}{4}) ?

    Bobak
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