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Math Help - Equation

  1. #1
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    Equation

    (3x +a) (bx-3)= 6x^2 +x-15

    The equation is true for all values of x, when a and b are constant. What is the value of ab?

    i did the foil method and came up with 10. Is that correct? or is it -10?
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  2. #2
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    Re: Equation

    +10 is correct
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  3. #3
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    Re: Equation

    Quote Originally Posted by tahirakw View Post
    (3x +a) (bx-3)= 6x^2 +x-15

    The equation is true for all values of x, when a and b are constant. What is the value of ab?

    i did the foil method and came up with 10. Is that correct? or is it -10?
    \displaystyle \begin{align*} (3x + a)(b\,x - 3) &= 3b\,x^2 + (ab - 9)x - 3a \end{align*}

    For this to be equal to \displaystyle \begin{align*} 6x^2 + x - 15 \end{align*} it requires that \displaystyle \begin{align*} ab - 9 = 1 \end{align*}, and so \displaystyle \begin{align*} ab = 10 \end{align*} as you found.
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