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Math Help - [SOLVED] cos(a+b)

  1. #1
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    [SOLVED] cos(a+b)

    if sin (a) =40/41, and a lies in quadrant 1
    and sin b = 40/41 and lies in quadrant II
    what is the exact value of cos(a+b)?
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  2. #2
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    Quote Originally Posted by So0pahBoy View Post
    if sin (a) =40/41, and a lies in quadrant 1
    and sin b = 40/41 and lies in quadrant II
    what is the exact value of cos(a+b)?
    HI

    \cos a =\frac{9}{41}

    \cos b=-\frac{9}{41}

    Now \cos (a+b)=\cos a\cos b-\sin a\sin b

    Substitute appropriately .
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  3. #3
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    Thanks, but, how did you find the cos?
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  4. #4
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    Quote Originally Posted by So0pahBoy View Post
    Thanks, but, how did you find the cos?
    Draw 2 triangles in the 1st and 2nd quadrant . Read the qusetion again to know which belongs to which . Then phythagoras would give the remaining side . In the 1st quadrant , everything is positive but in the 2nd , only sin is positive , cos is negative .
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  5. #5
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    ahh, okay, thanks!
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