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Thread: Trigonometry (cos)

  1. #1
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    Trigonometry (cos)

    Show $\displaystyle cos \theta + cos (\theta + \alpha) + cos (\theta + 2\alpha) = (2 cos \alpha + 1) cos (\theta + \alpha)$

    Answer:
    $\displaystyle cos \theta + cos (\theta + \alpha) + cos (\theta + 2 \alpha)$
    $\displaystyle =[cos (\theta + 2 \alpha) + cos \theta] + cos (\theta + \alpha)$
    $\displaystyle =2 cos (\theta + \alpha) cos \alpha + cos (\theta + \alpha)$
    $\displaystyle =(2 cos \alpha + 1)cos (\theta + \alpha)$

    My question is what happen between this step?
    $\displaystyle =2 cos (\theta + \alpha) cos \alpha + cos (\theta + \alpha)$
    $\displaystyle =(2 cos \alpha + 1)cos (\theta + \alpha)$
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  2. #2
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    $\displaystyle cos(\theta + \alpha)$ was factored out.
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  3. #3
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    please clarify your question. which step u didnot understand???
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  4. #4
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    Quote Originally Posted by Pulock2009 View Post
    please clarify your question. which step u didnot understand???
    I wanna know the formula between $\displaystyle =2 cos (\theta + \alpha) cos \alpha + cos (\theta + \alpha)$
    $\displaystyle =(2 cos \alpha + 1)cos (\theta + \alpha)$ how were they factorised?
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