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Math Help - Value of cos(2x-3pi/4).

  1. #1
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    Value of cos(2x-3pi/4).

    Sin(x)= -2/(5^0.5), 3pi/2<x<2pi

    then i have that cos(x)=+-(1-(-2/(5^0.5))^0.5=+-1/(5^0.5)
    and tan(x)=((-2/(5^0.5))/(1/(5^0.5)=2

    but what is the exaktly valus of this expression: cos(2x-3pi/4)?
    Last edited by mr fantastic; January 10th 2011 at 01:30 PM. Reason: Re-titled.
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  2. #2
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    Quote Originally Posted by paulaa View Post
    Sin(x)= -2/(5^0.5), 3pi/2<x<2pi

    then i have that cos(x)=+-(1-(-2/(5^0.5))^0.5=+-1/(5^0.5)
    and tan(x)=((-2/(5^0.5))/(1/(5^0.5)=2

    but what is the exaktly valus of this expression: cos(2x-3pi/4)?
    Get cos(2x-3pi/4) in terms of cos(x) using compound angle and double angle formulae. Then substitute the information you've been given.
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