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Solve -2x² - 8x = 0

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

x² +x += 0
−2x2−8x=0⇒x=−4, 0-2x^2 - 8x = 0 \Rightarrow x = -4,\ 0
Discriminant D = 64 · x ≈ -4, 0

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

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Graph of y = -2x² - 8x-6-5-4-3-2-112-30-20-1010x = -4x = 0vertexy = -2x² - 8x
Graph of y = -2x² - 8x. The curve crosses the x-axis at x = -4 and x = 0; the vertex is at (-2, 8).

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 = -8, c = 0.

    −2x2−8x=0-2x^2 - 8x = 0
  2. 2

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

    D=(−8)2−4⋅(−2)⋅0=64+0=64D = (-8)^2 - 4\cdot(-2)\cdot0 = 64 + 0 = 64
  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)=0-2\left(x + 4\right)\left(x\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=0⇒x=0x + 4 = 0 \Rightarrow x = -4\qquad x = 0 \Rightarrow x = 0
  6. 6

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=8±64−4x = \frac{-b \pm \sqrt{D}}{2a} = \frac{8 \pm \sqrt{64}}{-4}
  7. 7

    Simplify to get the solutions:

    x=−4orx=0x = -4\quad\text{or}\quad x = 0
  8. 8

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

LaTeX source of the formulas on this page
\[ -2x^2 - 8x = 0\ \Rightarrow\ x = -4,\ 0 \]
\[ -2x^2 - 8x = 0 \]
\[ D = (-8)^2 - 4\cdot(-2)\cdot0 = 64 + 0 = 64 \]
\[ -2\left(x + 4\right)\left(x\right) = 0 \]
\[ x + 4 = 0 \Rightarrow x = -4\qquad x = 0 \Rightarrow x = 0 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{8 \pm \sqrt{64}}{-4} \]
\[ x = -4\quad\text{or}\quad x = 0 \]

The parabola y = -2x² - 8x

xy
-5-10
-40
-36
-28
-16
00
1-10

Questions

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

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

Does -2x² - 8x = 0 factor?

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

What is the vertex of y = -2x² - 8x?

(-2, 8). The parabola opens downwards, so this is the maximum.

What are the sum and product of the roots?

Sum = −b/a = -4, product = c/a = 0.

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