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Math Help - I feel like an idiot for not being able to figure out this basic trig proplem, help?

  1. #1
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    I feel like an idiot for not being able to figure out this basic trig proplem, help?

    I got the stuff down cold but I cant seem to figure out this problem

    Side 1=7.21
    Side 2=7.21
    Side 3=10

    The question states: Using law of sines or law of cosines find all 3 angles.

    Its so easy but i keep blanking.
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  2. #2
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    Quote Originally Posted by john504 View Post
    I got the stuff down cold but I cant seem to figure out this problem

    Side a=7.21
    Side b=7.21
    Side c=10

    The question states: Using law of sines or law of cosines find all 3 angles.

    Its so easy but i keep blanking.
    I changed 1,2 and 3 to a,b and c respectively to make notation easier. In a triangle side a is opposite angle A and so on.

    Use the cos rule to find angle C:
    cos(C)=\frac{a^2+b^2-c^2}{2ab}

    Then you can use the sine rule rule to find A and/or B:

    \frac{a}{sin(A)} = \frac{b}{sin(B)} = \frac{c}{sin(C)}

    To find the last angle either use the sine rule again or (easier) take the other two angles from pi or 180 degrees


    ---------

    edit: Since this is an isosceles triangle side a = side b and so angle A = angle B

    Once you've found angle C you can do \frac{\pi - C}{2}
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  3. #3
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    Thanks, I got it now. I had to switch up angles b and c but i got it.
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