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Solve -2x² - 10x - 8 = 0

The solutions of -2x² - 10x - 8 = 0 are x = -4 or x = -1.

x² +x += 0
−2x2−10x−8=0⇒x=−4, −1-2x^2 - 10x - 8 = 0 \Rightarrow x = -4,\ -1
Discriminant D = 36 · x ≈ -4, -1

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

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Graph of y = -2x² - 10x - 8-6-5-4-3-2-11-25-20-15-10-55x = -4x = -1vertexy = -2x² - 10x - 8
Graph of y = -2x² - 10x - 8. The curve crosses the x-axis at x = -4 and x = -1; the vertex is at (-2.5, 4.5).

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 = -10, c = -8.

    −2x2−10x−8=0-2x^2 - 10x - 8 = 0
  2. 2

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

    D=(−10)2−4⋅(−2)⋅(−8)=100−64=36D = (-10)^2 - 4\cdot(-2)\cdot(-8) = 100 - 64 = 36
  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:

    −2(x+4)(x+1)=0-2\left(x + 4\right)\left(x + 1\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=−4x+1=0⇒x=−1x + 4 = 0 \Rightarrow x = -4\qquad x + 1 = 0 \Rightarrow x = -1
  6. 6

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=10±36−4x = \frac{-b \pm \sqrt{D}}{2a} = \frac{10 \pm \sqrt{36}}{-4}
  7. 7

    Simplify to get the solutions:

    x=−4orx=−1x = -4\quad\text{or}\quad x = -1
  8. 8

    Check by putting x = -4 back in: a·x² + b·x + c = -2·(16) + (-10)·(-4) + (-8) = 0 ✓

LaTeX source of the formulas on this page
\[ -2x^2 - 10x - 8 = 0\ \Rightarrow\ x = -4,\ -1 \]
\[ -2x^2 - 10x - 8 = 0 \]
\[ D = (-10)^2 - 4\cdot(-2)\cdot(-8) = 100 - 64 = 36 \]
\[ -2\left(x + 4\right)\left(x + 1\right) = 0 \]
\[ x + 4 = 0 \Rightarrow x = -4\qquad x + 1 = 0 \Rightarrow x = -1 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{10 \pm \sqrt{36}}{-4} \]
\[ x = -4\quad\text{or}\quad x = -1 \]

The parabola y = -2x² - 10x - 8

xy
-5.5-13.5
-4.5-3.5
-3.52.5
-2.54.5
-1.52.5
-0.5-3.5
0.5-13.5

Questions

What are the solutions of -2x² - 10x - 8 = 0?

The solutions of -2x² - 10x - 8 = 0 are x = -4 or x = -1.

Does -2x² - 10x - 8 = 0 factor?

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

What is the vertex of y = -2x² - 10x - 8?

(-5/2, 9/2). The parabola opens downwards, so this is the maximum.

What are the sum and product of the roots?

Sum = −b/a = -5, product = c/a = 4.

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