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For what values of m does the quadratic equation (m-7)x2 + (2m-1)x + (2m+3) = 0 have real roots?

Mathematics

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Answer

−1.8≤m≤11.8-1.8 \le m \le 11.8

Step-by-step solution

6 steps
  1. Identify the coefficients of the quadratic equation.

    The given equation is (m−7)x2+(2m−1)x+(2m+3)=0(m-7)x^2 + (2m-1)x + (2m+3) = 0.
    This is in the form Ax2+Bx+C=0Ax^2 + Bx + C = 0, where:
    A=m−7A = m-7
    B=2m−1B = 2m-1
    C=2m+3C = 2m+3

  2. Apply the condition for real roots.

    For a quadratic equation to have real roots, the discriminant Δ=B2−4AC\Delta = B^2 - 4AC must be greater than or equal to zero.

    Δ=(2m−1)2−4(m−7)(2m+3)≥0\Delta = (2m-1)^2 - 4(m-7)(2m+3) \ge 0
  3. Expand and simplify the inequality.

    Expand the terms:

    (2m−1)2=(2m)2−2(2m)(1)+12=4m2−4m+1(2m-1)^2 = (2m)^2 - 2(2m)(1) + 1^2 = 4m^2 - 4m + 1 4(m−7)(2m+3)=4(2m2+3m−14m−21)=4(2m2−11m−21)=8m2−44m−844(m-7)(2m+3) = 4(2m^2 + 3m - 14m - 21) = 4(2m^2 - 11m - 21) = 8m^2 - 44m - 84

    Substitute these back into the inequality:

    (4m2−4m+1)−(8m2−44m−84)≥0(4m^2 - 4m + 1) - (8m^2 - 44m - 84) \ge 0 4m2−4m+1−8m2+44m+84≥04m^2 - 4m + 1 - 8m^2 + 44m + 84 \ge 0

    Combine like terms:

    −4m2+40m+85≥0-4m^2 + 40m + 85 \ge 0

    Multiply the entire inequality by −1-1 and reverse the inequality sign:

    4m2−40m−85≤04m^2 - 40m - 85 \le 0
  4. Find the roots of the quadratic equation 4m2−40m−85=04m^2 - 40m - 85 = 0.

    Using the quadratic formula m=−b±b2−4ac2am = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}:

    m=−(−40)±(−40)2−4(4)(−85)2(4)m = \frac{-(-40) \pm \sqrt{(-40)^2 - 4(4)(-85)}}{2(4)} m=40±1600+13608m = \frac{40 \pm \sqrt{1600 + 1360}}{8} m=40±29608m = \frac{40 \pm \sqrt{2960}}{8}

    Calculate the value of 2960\sqrt{2960}:

    2960≈54.40588\sqrt{2960} \approx 54.40588

    Now find the two roots:

    m1=40−54.405888=−14.405888≈−1.8007m_1 = \frac{40 - 54.40588}{8} = \frac{-14.40588}{8} \approx -1.8007 m2=40+54.405888=94.405888≈11.8007m_2 = \frac{40 + 54.40588}{8} = \frac{94.40588}{8} \approx 11.8007
  5. Determine the range of mm and consider the case where A=0A=0.

    Since the quadratic 4m2−40m−854m^2 - 40m - 85 opens upwards (coefficient of m2m^2 is positive), the inequality 4m2−40m−85≤04m^2 - 40m - 85 \le 0 holds for values of mm between its roots.
    So, −1.8007≤m≤11.8007-1.8007 \le m \le 11.8007.

    We also need to consider the case where A=m−7=0A = m-7 = 0, which means m=7m=7. If m=7m=7, the original equation becomes 13x+17=013x + 17 = 0, which is a linear equation with one real root x=−1713x = -\frac{17}{13}. Since m=7m=7 results in a real root, it is included in the range. The value m=7m=7 falls within the calculated interval.

  6. Round the values to one decimal place.

    m1≈−1.8m_1 \approx -1.8
    m2≈11.8m_2 \approx 11.8

    The range of values for mm is −1.8≤m≤11.8\boxed{-1.8 \le m \le 11.8}.

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