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Solve -2x² - 11x - 12 = 0

The solutions of -2x² - 11x - 12 = 0 are x = -4 or x = -3/2.

x² +x += 0
−2x2−11x−12=0⇒x=−4, −32-2x^2 - 11x - 12 = 0 \Rightarrow x = -4,\ -\frac{3}{2}
Discriminant D = 25 · x ≈ -4, -1.5

Change a, b or c (whole numbers, a not 0) and the exact solutions follow.

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Graph of y = -2x² - 11x - 12-6-5-4-3-2-11-25-20-15-10-55x = -4x = -1.5vertexy = -2x² - 11x - 12
Graph of y = -2x² - 11x - 12. The curve crosses the x-axis at x = -4 and x = -1.5; the vertex is at (-2.75, 3.125).

How to work it out

  1. 1

    Write it in standard form a x² + b x + c = 0 and read off the coefficients: a = -2, b = -11, c = -12.

    −2x2−11x−12=0-2x^2 - 11x - 12 = 0
  2. 2

    Work out the discriminant D = b² − 4ac. Its sign tells you how many real solutions there are.

    D=(−11)2−4⋅(−2)⋅(−12)=121−96=25D = (-11)^2 - 4\cdot(-2)\cdot(-12) = 121 - 96 = 25
  3. 3

    D is positive, so there are two different real solutions.

  4. 4

    Method 1, factoring. The roots are rational, so the left side factors:

    −(x+4)(2x+3)=0-\left(x + 4\right)\left(2x + 3\right) = 0
  5. 5

    A product is zero when one of its factors is zero, so set each factor equal to 0 and solve:

    x+4=0⇒x=−42x+3=0⇒x=−32x + 4 = 0 \Rightarrow x = -4\qquad 2x + 3 = 0 \Rightarrow x = -\frac{3}{2}
  6. 6

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=11±25−4x = \frac{-b \pm \sqrt{D}}{2a} = \frac{11 \pm \sqrt{25}}{-4}
  7. 7

    Simplify to get the solutions:

    x=−4orx=−32x = -4\quad\text{or}\quad x = -\frac{3}{2}
  8. 8

    Check by putting x = -4 back in: a·x² + b·x + c = -2·(16) + (-11)·(-4) + (-12) = 0 ✓

LaTeX source of the formulas on this page
\[ -2x^2 - 11x - 12 = 0\ \Rightarrow\ x = -4,\ -\frac{3}{2} \]
\[ -2x^2 - 11x - 12 = 0 \]
\[ D = (-11)^2 - 4\cdot(-2)\cdot(-12) = 121 - 96 = 25 \]
\[ -\left(x + 4\right)\left(2x + 3\right) = 0 \]
\[ x + 4 = 0 \Rightarrow x = -4\qquad 2x + 3 = 0 \Rightarrow x = -\frac{3}{2} \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{11 \pm \sqrt{25}}{-4} \]
\[ x = -4\quad\text{or}\quad x = -\frac{3}{2} \]

The parabola y = -2x² - 11x - 12

xy
-5.75-14.875
-4.75-4.875
-3.751.125
-2.753.125
-1.751.125
-0.75-4.875
0.25-14.875

Questions

What are the solutions of -2x² - 11x - 12 = 0?

The solutions of -2x² - 11x - 12 = 0 are x = -4 or x = -3/2.

Does -2x² - 11x - 12 = 0 factor?

Yes: -1(x+4)(2x+3) = 0.

What is the vertex of y = -2x² - 11x - 12?

(-11/4, 25/8). The parabola opens downwards, so this is the maximum.

What are the sum and product of the roots?

Sum = −b/a = -11/2, product = c/a = 6.

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