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Given f(x) = ax4 + 7x3 + x2 + bx - 3, if (x-1) and (x+1) are factors, find a and b, and then find the set of real values of x for which f(x) > 0.

Mathematics

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Answer

a=2,b=−7a=2, b=-7

x∈(−∞,−3)∪(−1,−12)∪(1,∞)x \in (-\infty, -3) \cup \left(-1, -\frac{1}{2}\right) \cup (1, \infty)

Step-by-step solution

4 steps
  1. Use the Factor Theorem to find aa and bb.

    The polynomial is f(x)=ax4+7x3+x2+bx−3f(x) = ax^4 + 7x^3 + x^2 + bx - 3.
    Given that (x−1)(x-1) and (x+1)(x+1) are factors of f(x)f(x), we know that f(1)=0f(1)=0 and f(−1)=0f(-1)=0.

    For f(1)=0f(1)=0:

    a(1)4+7(1)3+(1)2+b(1)−3=0a(1)^4 + 7(1)^3 + (1)^2 + b(1) - 3 = 0 a+7+1+b−3=0a + 7 + 1 + b - 3 = 0 a+b+5=0a + b + 5 = 0 a+b=−5(∗)a + b = -5 \quad (*)

    For f(−1)=0f(-1)=0:

    a(−1)4+7(−1)3+(−1)2+b(−1)−3=0a(-1)^4 + 7(-1)^3 + (-1)^2 + b(-1) - 3 = 0 a(1)+7(−1)+1−b−3=0a(1) + 7(-1) + 1 - b - 3 = 0 a−7+1−b−3=0a - 7 + 1 - b - 3 = 0 a−b−9=0a - b - 9 = 0 a−b=9(∗∗)a - b = 9 \quad (**)
  2. Solve the system of linear equations for aa and bb.

    We have:

    1. a+b=−5a + b = -5
    2. a−b=9a - b = 9

    Add equation (1) and equation (2):

    (a+b)+(a−b)=−5+9(a+b) + (a-b) = -5 + 9 2a=42a = 4 a=2a = 2

    Substitute a=2a=2 into equation (1):

    2+b=−52 + b = -5 b=−5−2b = -5 - 2 b=−7b = -7

    The values of the constants are a=2a=2 and b=−7b=-7.

    a=2,b=−7\boxed{a=2, b=-7}
  3. Find the set of real values of xx for which f(x)>0f(x) > 0.

    Substitute a=2a=2 and b=−7b=-7 into f(x)f(x):

    f(x)=2x4+7x3+x2−7x−3f(x) = 2x^4 + 7x^3 + x^2 - 7x - 3

    Since (x−1)(x-1) and (x+1)(x+1) are factors, their product (x−1)(x+1)=x2−1(x-1)(x+1) = x^2-1 is also a factor.
    Perform polynomial long division to find the other factor:

    (2x4+7x3+x2−7x−3)÷(x2−1)(2x^4 + 7x^3 + x^2 - 7x - 3) \div (x^2 - 1) \multicolumn2r2x2+7x+3\cline2−6x2−12x4+7x3+x2−7x−3\multicolumn2r−(2x4−2x2)\cline2−4\multicolumn2r0+7x3+3x2−7x\multicolumn2r−(7x3−7x)\cline3−5\multicolumn2r0+3x20−3\multicolumn2r−(3x2−3)\cline4−6\multicolumn2r000\begin{array}{c|cc cc cc} \multicolumn{2}{r}{2x^2} & +7x & +3 \\ \cline{2-6} x^2-1 & 2x^4 & +7x^3 & +x^2 & -7x & -3 \\ \multicolumn{2}{r}{-(2x^4} & & -2x^2) \\ \cline{2-4} \multicolumn{2}{r}{0} & +7x^3 & +3x^2 & -7x \\ \multicolumn{2}{r}{} & -(7x^3 & & -7x) \\ \cline{3-5} \multicolumn{2}{r}{} & 0 & +3x^2 & 0 & -3 \\ \multicolumn{2}{r}{} & & -(3x^2 & & -3) \\ \cline{4-6} \multicolumn{2}{r}{} & & 0 & 0 & 0 \\ \end{array}

    So, f(x)=(x2−1)(2x2+7x+3)f(x) = (x^2-1)(2x^2+7x+3).
    Factor the quadratic term 2x2+7x+32x^2+7x+3:

    2x2+7x+3=2x2+x+6x+3=x(2x+1)+3(2x+1)=(x+3)(2x+1)2x^2+7x+3 = 2x^2+x+6x+3 = x(2x+1)+3(2x+1) = (x+3)(2x+1)

    Thus, the fully factored form of f(x)f(x) is:

    f(x)=(x−1)(x+1)(x+3)(2x+1)f(x) = (x-1)(x+1)(x+3)(2x+1)
  4. Determine the intervals where f(x)>0f(x) > 0.

    The roots of f(x)f(x) are the values of xx where f(x)=0f(x)=0:
    x−1=0  ⟹  x=1x-1=0 \implies x=1
    x+1=0  ⟹  x=−1x+1=0 \implies x=-1
    x+3=0  ⟹  x=−3x+3=0 \implies x=-3
    2x+1=0  ⟹  x=−122x+1=0 \implies x = -\frac{1}{2}

    Order the roots on a number line: −3,−1,−12,1-3, -1, -\frac{1}{2}, 1.
    These roots divide the number line into five intervals. We test the sign of f(x)f(x) in each interval:

    • For x<−3x < -3 (e.g., x=−4x=-4): f(−4)=(−5)(−3)(−1)(−7)=105>0f(-4) = (-5)(-3)(-1)(-7) = 105 > 0

    • For −3<x<−1-3 < x < -1 (e.g., x=−2x=-2): f(−2)=(−3)(−1)(1)(−3)=−9<0f(-2) = (-3)(-1)(1)(-3) = -9 < 0

    • For −1<x<−12-1 < x < -\frac{1}{2} (e.g., x=−0.75x=-0.75): f(−0.75)=(−1.75)(0.25)(2.25)(−0.5)≈0.49>0f(-0.75) = (-1.75)(0.25)(2.25)(-0.5) \approx 0.49 > 0

    • For −12<x<1-\frac{1}{2} < x < 1 (e.g., x=0x=0): f(0)=(−1)(1)(3)(1)=−3<0f(0) = (-1)(1)(3)(1) = -3 < 0

    • For x>1x > 1 (e.g., x=2x=2): f(2)=(1)(3)(5)(5)=75>0f(2) = (1)(3)(5)(5) = 75 > 0

    We are looking for f(x)>0f(x) > 0. This occurs in the intervals (−∞,−3)(-\infty, -3), (−1,−12)(-1, -\frac{1}{2}), and (1,∞)(1, \infty).
    The set of real values of xx for which f(x)>0f(x) > 0 is:

    x∈(−∞,−3)∪(−1,−12)∪(1,∞)\boxed{x \in (-\infty, -3) \cup \left(-1, -\frac{1}{2}\right) \cup (1, \infty)}
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