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Math Help - Obtuse Angle and Sine

  1. #1
    Junior Member
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    Smile Obtuse Angle and Sine

    Hello!

    This is a question I couldn't solve.

    If A is an obtuse angle in a triangle and sin A = 5/13, calculate the exact value of sin 2A.

    I would appreciate your help,
    Amine
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  2. #2
    MHF Contributor
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    Quote Originally Posted by Aminekhadir View Post
    Hello!

    This is a question I couldn't solve.

    If A is an obtuse angle in a triangle and sin A = 5/13, calculate the exact value of sin 2A.

    I would appreciate your help,
    Amine
    HI

    A would be in the second quadrant since sin A is positive and its obtuse .

    Then by phythagoras , the other side would be 12

    so cos A = - 12/13 (negative because cos is negative in 2nd qua)

    sin 2A = 2 sin A cos A=2(5/13)(-12/13)
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