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Math Help - finding integers

  1. #1
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    Smile finding integers

    (px + 2)2 = 9x2 +qx + 4

    Where p and q are positive integers find the values of p and q

    not quite sure how to go about this one

    thanks for any help
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by pop_91 View Post
    (px + 2)2 = 9x2 +qx + 4

    Where p and q are positive integers find the values of p and q

    not quite sure how to go about this one

    thanks for any help
    expand the left hand side and compare coefficients

    an alternate way would be to complete the square of the right hand side, and then you will clearly see what p and q are
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  3. #3
    Moo
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    Hello,
    Quote Originally Posted by pop_91 View Post
    (px + 2)2 = 9x2 +qx + 4

    Where p and q are positive integers find the values of p and q

    not quite sure how to go about this one

    thanks for any help
    Is it (px+2)^2=9x^2+qx+4 ?

    The rough way is to develop the LHS and recognize the coefficients.

    Another way is to find q such that 9x+qx+4 is a perfect square, since the LHS is a perfect square.
    Note that it is equal to (3x)+qx+2
    So it would be (3x+2)=9x+12x+4.

    And 3x+2=px+2
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  4. #4
    Super Member Matt Westwood's Avatar
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    (px + 2)^2 = 9x^2 +qx + 4

    First you can multiply out and simplify:

    p^2x^2 + 4px + 4 = 9x^2 +qx + 4

    (p^2 - 9)x^2 + (4p - q)x = 0

    As this is supposed to be an identity for all x, you can equate powers of x on both sides. As the RHS is zero (because everything's been gathered to the left), you can say:

    Coefficients of x^2: p^2 - 9 = 0

    Coefficients of x: 4p - q = 0

    take it away, maestro ...
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  5. #5
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    Is the 9x2 9 \cdot 2 or 9x \cdot 2 ?

    I'm going to assume it's the latter, so you have

    2(px + 2) = 18x + qx + 4

    2px + 4 = 18x + qx + 4

    2px = 18x + qx

    2px - qx = 18x

    x(2p - q) = 18x

    2p - q = 18

    So basically any positive integers for p and q such that the above equation is true will suffice. (p, q) = (10, 2) is one such solution (and provides the lowest possible values for p and q).
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  6. #6
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    supposed to be
    (px + 2) squared = 9x squared + qx + 4
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