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Math Help - Finding value of c in a quadratic

  1. #1
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    Smile Finding value of c in a quadratic

    The remainder when -4x^2 + 2x + 7 is divided by (x - c) is -5. Find a possible whole number value of c.

    This is what I tried to do:

    P(c)= -4c^2 + 2c + 7
    -4c^2 + 2c + 7 = -5
    -4c^2 + 2c = -12

    Is this right? If so, what do I do next?
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  2. #2
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    c(-2c+1)=-6\rightarrow c=\pm 1, \pm 2, \pm 3, \pm 6 and you should check that 1-2c is correct... But if you solve the quadratic you can see that there are so real solutions for c...
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  3. #3
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    Unhappy

    Sorry...I don't understand.
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  4. #4
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    Sorry, it's my fault I were wrong. Just solve it as a quadratic equation: -4c^2 + 2c = -12\Rightarrow -2c^2+c+6=0\rightarrow c_{1,2}=\frac {-1\pm\sqrt{1^2-4\cdot (-2)\cdot 6}}{-4}= \frac {-1\pm 7}{-4}\rightarrow c_1=2\quad c_2=-\frac 32
    Only 2 is a whole number. Try it: -4x^2 + 2x + 7=(x-2)(-4x-6)-5. Yeah
    Last edited by james_bond; April 7th 2009 at 07:06 AM.
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