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Math Help - need help with homework

  1. #1
    Newbie Viky's Avatar
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    Post need help with homework

    if sin 70=t write the following in terms of t:

    1] cos 160
    2] cos 70
    3] sin 140

    you need to use the expansion method right?
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  2. #2
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    For #1, use the identity \cos (x + 90^{\circ}) = -\sin x.

    For #2, use the Pythagorean identity \sin^2 x + \cos^2 x = 1.

    For #3, use the double-angle identity \sin (2x) = 2\sin x \cos x.

    In all three cases, substitute 70 for x.


    01
    Last edited by yeongil; July 20th 2009 at 08:35 AM.
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  3. #3
    Newbie Viky's Avatar
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    could you give me an example for one of them?
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  4. #4
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    \begin{aligned}<br />
\cos 160^{\circ}  &= \cos (70^{\circ} + 90^{\circ}) \\<br />
&= -\sin 70^{\circ} \\<br />
&= -t<br />
\end{aligned}


    01
    Last edited by yeongil; July 20th 2009 at 08:37 AM.
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  5. #5
    Newbie Viky's Avatar
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    wow i don't know why i confused myself with this stuff. Thanks alot i understand now
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  6. #6
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    EDIT: I am sleep deprived. Sigh.
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