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Math Help - How does cosx^2 -1 = -sinx^2? Q concerning limits.

  1. #1
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    How does cosx^2 -1 = -sinx^2? Q concerning limits.



    How does that work? I've tried it with various values on the calculator and it's not equal. eg cos4 - 1 is not equal to -sin4 (if theta = 2). If neccesary, I can post the entire question. It concerns limits.
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  2. #2
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    Re: How does cosx^2 -1 = -sinx^2? Q concerning limits.

    Are you squaring the trig. functions when you test with your calculator?
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  3. #3
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    Re: How does cosx^2 -1 = -sinx^2? Q concerning limits.

    I've got an old fx-82. I don't have any option to square the actual trig function, but I should be able to square the x value right?
    let theta = x (because it's quicker to write)
    so let x = 2

    cos^2(x) -1 = cos4 -1 = -0.002
    -sin^2(x) = -sin4 = -0.07

    No idea what I am doing wrong..

    edit: Actually, it should be (cosx)^2 right? not what I am doing. I think I got it.
    Last edited by lukasaurus; May 2nd 2013 at 03:57 PM.
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    Re: How does cosx^2 -1 = -sinx^2? Q concerning limits.

    You may be confusing cos^2(x) with cos(x^2).


    If x= 2 radians, then cos(x)= cos(2)= -0.416147 so that cos^2(2)= 0.17318.

    But cos(x^2)= cos(4)= -0.6536436.
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