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Math Help - Cosine law

  1. #1
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    Cosine law

    Q : if Cos B / a = Cos A / b
    Prove that : a=b and c^2 = a^2 + b^2
    Last edited by mr fantastic; September 7th 2011 at 01:53 PM. Reason: Title.
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  2. #2
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    Re: want help in cosine law

    Quote Originally Posted by mido22 View Post
    Q : if Cos B / a = Cos A / b
    Prove that : a=b and c^2 = a^2 + b^2
    What have you tried? You could start by using the cosine rule to express cos A and cos B in terms of a, b and c. Then try to simplify the resulting equation.

    Edit. After checking the question, I think that you have stated it wrongly. It should say

    Prove that : a=b or c^2=a^2+b^2.
    Last edited by Opalg; September 6th 2011 at 11:49 PM.
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    Re: want help in cosine law

    yes it is or not and i made alot of tries but all reach to same result :
    b^2 ( c^2 - b^2 ) = a^2 ( c^2 - a^2 )
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  4. #4
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    Re: want help in cosine law

    Quote Originally Posted by mido22 View Post
    yes it is or not and i made alot of tries but all reach to same result :
    b^2 ( c^2 - b^2 ) = a^2 ( c^2 - a^2 )
    Can you say a=b from this?
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  5. #5
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    Re: want help in cosine law

    no i can't say so becz a!=b
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    Re: want help in cosine law

    Quote Originally Posted by mido22 View Post
    yes it is or not and i made alot of tries but all reach to same result :
    b^2 ( c^2 - b^2 ) = a^2 ( c^2 - a^2 )
    b^2(c^2-b^2) = a^2(c^2-a^2)

    b^2c^2 - b^4 = a^2c^2 - a^4

    b^2c^2 - a^2c^2 = b^4 - a^4

    c^2(b^2 - a^2) = (b^2 - a^2)(b^2 + a^2)

    since a \ne b ...

    c^2 = b^2 + a^2
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  7. #7
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    Re: want help in cosine law

    and how can i get a=b from same problem
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