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Math Help - HELP deriving the cosine rule?

  1. #1
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    HELP deriving the cosine rule?

    Hello there, one of my text books follows that:

    a2 = (c-bcosA)^2 + (bsinA)^2

    leads to:

    a^2 = c2 -2cb cosA +b2Cos2A +b2sin2A

    Please can someone please help as this is doing my head in! many thanks

    i start by expanding out the brackets of the first term and distributing, but getting confused.. where does the -2cb come from!? ... thanks in advance!
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  2. #2
    MHF Contributor Siron's Avatar
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    Re: HELP deriving the cosine rule?

    Use: (a-b)^2=a^2-2.a.b+b^2 so you get:
    (c-b.cos(A))^2=c^2-2.c.b.cos(A)+b^2.cos^2(A)
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