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Math Help - Proving an equation.

  1. #1
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    Proving an equation.

    1) Prove that if PQ=C and P+Q=B then:

    a) X + bx + c = (x+p)(x+q)
    b) X - bx - c = (x-p)(x+q)

    And after answering it can you please explain the procedure :/ Thank you
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  2. #2
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    Quote Originally Posted by Ineedsomehelp View Post
    1) Prove that if PQ=C and P+Q=B then:

    a) X + bx + c = (x+p)(x+q)
    b) X - bx - c = (x-p)(x+q)

    And after answering it can you please explain the procedure :/ Thank you
    Just do the multiplication on the right. By the "distributive" property, (x+ p)(x+ q)= (x+ p)x+ (x+ p)q. Using the "distributive" property on each of those, (x+ p)(x+ q)= x(x)+ px+ xq+ pq= x^2+ (p+q)x+ pq. Since p+ q= B and pq= C, (x+ p)(x+ q)= x^2+ (p+q)x+ pq=  x^2+ Bx+ C.

    You try the second one.

    By the way, it is a bad idea to use capital letters and small letters to mean the same thing "P" and "p" are different symbols and imply possibly different values.
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  3. #3
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    :/ I don't totally get it. Just wondering if you could go in a little bit of further explanation off course if you don't mind Thank you.
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  4. #4
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    Quote Originally Posted by HallsofIvy View Post
    Just do the multiplication on the right.
    (x+ p)(x+ q)= (x+ p)x+ (x+ p)q.
    (x+ p)(x+ q)= x(x)+ px+ xq+ pq= x^2+ (p+q)x+ pq. Since p+ q= B and pq= C, (x+ p)(x+ q)= x^2+ (p+q)x+ pq=  x^2+ Bx+ C.

    ...
    Quote Originally Posted by Ineedsomehelp View Post
    :/ I don't totally get it. <=== are you sure?
    Just wondering if you could go in a little bit of further explanation off course if you don't mind Thank you.
    HallsofIvy took the RHS of the equation and transformed it into the LHs of your equation by comparison of the coefficients.

    Of course you can factorize the LHS and show that you have reached the RHS:

    \begin{array}{c}x^2+bx +c &=& x^2+bx +\frac14 b^2 - \frac14b^2 + c \\ =(x+\frac12b)^2-(\frac14b^2-c) \\ &=& (x+\frac12b)^2-\left(\sqrt{\frac14b^2-c}\right)^2  \end{array}

    This is a difference of squares which can be factored to:

    (x+\frac12b)^2-\left(\sqrt{\frac14b^2-c}\right)^2 = \left(x+\underbrace{\frac12b + \sqrt{\frac14b^2-c}}_{p}  \right) \left( x+\underbrace{\frac12b - \sqrt{\frac14b^2-c}}_{q} \right)
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  5. #5
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    ohh thanks well I pretty much get it now ^^ Ty for helping
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