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Solve x² + 8x = 0

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

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

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

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Graph of y = x² + 8x-10-8-6-4-22-20-101020x = 0x = -8vertexy = x² + 8x
Graph of y = x² + 8x. The curve crosses the x-axis at x = 0 and x = -8; the vertex is at (-4, -16).

How to work it out

  1. 1

    Write it in standard form a x² + b x + c = 0 and read off the coefficients: a = 1, b = 8, c = 0.

    x2+8x=0x^2 + 8x = 0
  2. 2

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

    D=82−4⋅1⋅0=64+0=64D = 8^2 - 4\cdot1\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:

    (x)(x+8)=0\left(x\right)\left(x + 8\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=0⇒x=0x+8=0⇒x=−8x = 0 \Rightarrow x = 0\qquad x + 8 = 0 \Rightarrow x = -8
  6. 6

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=−8±642x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-8 \pm \sqrt{64}}{2}
  7. 7

    Simplify to get the solutions:

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

    Check by putting x = 0 back in: a·x² + b·x + c = 1·(0) + 8·(0) + 0 = 0 ✓

LaTeX source of the formulas on this page
\[ x^2 + 8x = 0\ \Rightarrow\ x = 0,\ -8 \]
\[ x^2 + 8x = 0 \]
\[ D = 8^2 - 4\cdot1\cdot0 = 64 + 0 = 64 \]
\[ \left(x\right)\left(x + 8\right) = 0 \]
\[ x = 0 \Rightarrow x = 0\qquad x + 8 = 0 \Rightarrow x = -8 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-8 \pm \sqrt{64}}{2} \]
\[ x = 0\quad\text{or}\quad x = -8 \]

The parabola y = x² + 8x

xy
-7-7
-6-12
-5-15
-4-16
-3-15
-2-12
-1-7

Questions

What are the solutions of x² + 8x = 0?

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

Does x² + 8x = 0 factor?

Yes: (x)(x+8) = 0.

What is the vertex of y = x² + 8x?

(-4, -16). The parabola opens upwards, so this is the minimum.

What are the sum and product of the roots?

Sum = −b/a = -8, product = c/a = 0.

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