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Math Help - trig sine angle

  1. #1
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    trig sine angle

    i need help on this question:
    Find the exact value of sin 67.5 , i know that 2a is 135. and the terminal angle is 45 degree.

    I also need help on this, Express Pie / 2 in terms of cos A.
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  2. #2
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    Quote Originally Posted by lickman View Post
    i need help on this question:
    Find the exact value of sin 67.5 , i know that 2a is 135. and the terminal angle is 45 degree.
    [snip]
    \cos(2 \times 67.5^o) = \cos (135^o) = - \cos(45^o).

    But from the double angle formula, \cos(2 \times 67.5^o) = 1 - 2 \sin^2 (67.5^o).

    Therefore ....
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  3. #3
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    arent they asking for sin?? the exact value of sin 67.5, so wouldnt u use
    Sin(2 x 67.5) = 2(1/square Root 2) (1/square root 2)
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    Quote Originally Posted by lickman View Post
    arent they asking for sin?? the exact value of sin 67.5, so wouldnt u use
    Sin(2 x 67.5) = 2(1/square Root 2) (1/square root 2)
    And how is that going to give you the value of sin(67.5), pray tell?

    Solve -\sqrt{2} = 1 - 2 \sin^2 (67.5^o) as I suggested.
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