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Find the values of p and q If (x2 + x - 2) is a factor of f(x) = x3 + px2 + 2x + q.

Mathematics

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Answer

p=5,q=−8\mathbf{p=5, q=-8}

Step-by-step solution

4 steps

To solve Question 1.3, we will use the Factor Theorem. If (x2+x−2)(x^2 + x - 2) is a factor of f(x)f(x), then the roots of (x2+x−2)=0(x^2 + x - 2) = 0 must also be roots of f(x)=0f(x) = 0.

  1. Find the roots of the quadratic factor.

    Factorize the quadratic expression x2+x−2x^2 + x - 2:

    x2+x−2=0x^2 + x - 2 = 0 (x+2)(x−1)=0(x+2)(x-1) = 0

    The roots are x=−2x = -2 and x=1x = 1.

  2. Use the Factor Theorem to set up equations.

    Since (x+2)(x+2) and (x−1)(x-1) are factors of f(x)=x3+px2+2x+qf(x) = x^3 + px^2 + 2x + q, we know that f(−2)=0f(-2) = 0 and f(1)=0f(1) = 0.

    For x=1x=1:

    f(1)=(1)3+p(1)2+2(1)+q=0f(1) = (1)^3 + p(1)^2 + 2(1) + q = 0 1+p+2+q=01 + p + 2 + q = 0 p+q+3=0p + q + 3 = 0 p+q=−3(Equation 1)p + q = -3 \quad \text{(Equation 1)}

    For x=−2x=-2:

    f(−2)=(−2)3+p(−2)2+2(−2)+q=0f(-2) = (-2)^3 + p(-2)^2 + 2(-2) + q = 0 −8+4p−4+q=0-8 + 4p - 4 + q = 0 4p+q−12=04p + q - 12 = 0 4p+q=12(Equation 2)4p + q = 12 \quad \text{(Equation 2)}
  3. Solve the system of linear equations for pp and qq.

    We have the system:

    1. p+q=−3p + q = -3
    2. 4p+q=124p + q = 12

    Subtract Equation 1 from Equation 2:

    (4p+q)−(p+q)=12−(−3)(4p + q) - (p + q) = 12 - (-3) 4p−p+q−q=12+34p - p + q - q = 12 + 3 3p=153p = 15 p=153p = \frac{15}{3} p=5p = 5
  4. Substitute the value of pp into Equation 1 to find qq.

    p+q=−3p + q = -3 5+q=−35 + q = -3 q=−3−5q = -3 - 5 q=−8q = -8

    The values are p=5,q=−8\mathbf{p=5, q=-8}.

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