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Math Help - Trig problem help plzzz

  1. #1
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    Exclamation Trig problem help plzzz

    Hi so i have this trig problem for homework and I can't get it, can you please help?
    Here it is:
    If A is an obtuse angle in a triangle and sinA= 5/13, calculate the exact balue of sin2A.
    Could you please do it step by step.
    Thank you!!
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  2. #2
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    Quote Originally Posted by kaleyy =] View Post
    Hi so i have this trig problem for homework and I can't get it, can you please help?
    Here it is:
    If A is an obtuse angle in a triangle and sinA= 5/13, calculate the exact balue of sin2A.
    Could you please do it step by step.
    Thank you!!
    You can construct a triangle with an angle A and opposite side 5 with hypotenuse 13.

    You can then use Pythagoras Theorem to find the missing side which turns out to be 12.

    We can then say cos(A) = adj/hyp = 12/13

    sin(2A) = 2sinAcosA = 2 \times \frac{5}{13} \times \frac{12}{13} = \frac{120}{169}

    I'm not too sure how the quadrants come into it so you may want to wait for someone more experienced with that part
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