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Math Help - rational equations

  1. #1
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    rational equations

    Determine the point of intersection of the following two rational functions, algebraically.

    r(x)= x/(3x^2+3) s(x)= 1/(2x^2-8)

    Thank you
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  2. #2
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    Quote Originally Posted by johett View Post
    Determine the point of intersection of the following two rational functions, algebraically.

    r(x)= x/(3x^2+3) s(x)= 1/(2x^2-8)

    Thank you
    The point of intersection is going to be defined by r(x) = s(x) for some values of x. So:
    \frac{x}{3x^2 + 3} = \frac{1}{2x^2 - 8}

    Cross multiply (or, more precisely, multiply both sides by (3x^2 + 3)(2x^2 - 8))

    x(2x^2 - 8) = 3x^2 + 3

    2x^3 - 3x^2 - 8x - 3 = 0

    You can use the rational root theorem to get the solutions. I get x = -1, -1/2, 3.

    -Dan
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  3. #3
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    Algebra and cubics

    First, note where the two functions might not be defined: s(x) is undefined for x=\pm2, so those points cannot be intersections.
    Then we set r(x)=s(x):
    \frac{x}{3x^2+3}=\frac{1}{2x^2-8}
    Multiply both sides by the product of the denominators and cancel:
    x(2x^2-8)=1(3x^2+3)
    Expand, and use algebra to move all terms to one side:
    2x^3-3x^2-8x-3=0
    This is a cubic equation, and there are methods of solving it. However, once you find one root, r_1, you can factor out the term (x-r_1) from the polynomial to obtain a quadratic which will tell you if there are two other real roots, and what they are (if they exist). Using the Rational Root Theorem can help you find that first root.

    --Kevin C.
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