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Homework Help: Unknown variables

  1. Sep 14, 2004 #1
    if f(x)=x^3+Ax^2+Bx-3
    if f(1)=4
    if f(-1)=-6

    what is 2A+B?

    1^3+A(1)^2+B(1)-3=4
    A+B-3=4
    A+B=7

    (-1)^3+A(-1)^2+B(-1)-3=-6
    -1+A-B-3=-6
    A+B=-2

    A=B-2
    B-2+B=7
    B=4.5

    B=A+2
    A+A+2=7
    A=2.5

    2A+B=
    2(2.5)+4.5=9.5

    The multiple choices are: (A)12 (B)8 (C) 0 (D) -2 (E) cannot be determined

    what am I doing wrong?
     
  2. jcsd
  3. Sep 14, 2004 #2
    This should have gone like this:
    13 + A(1)2 + B(1) -3 = 4
    A + B -2 = 4
    A + B = 6

    How did you get A + B = -2 from -1+A-B-3 = -6? The final equation should have been A - B = -2.
     
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