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Solve x² + 2x - 8 = 0

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

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

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

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Graph of y = x² + 2x - 8-6-4-224-10-55101520x = 2x = -4vertexy = x² + 2x - 8
Graph of y = x² + 2x - 8. The curve crosses the x-axis at x = 2 and x = -4; the vertex is at (-1, -9).

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 = 2, c = -8.

    x2+2x−8=0x^2 + 2x - 8 = 0
  2. 2

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

    D=22−4⋅1⋅(−8)=4+32=36D = 2^2 - 4\cdot1\cdot(-8) = 4 + 32 = 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:

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

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=−2±362x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-2 \pm \sqrt{36}}{2}
  7. 7

    Simplify to get the solutions:

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

    Check by putting x = 2 back in: a·x² + b·x + c = 1·(4) + 2·(2) + (-8) = 0 ✓

LaTeX source of the formulas on this page
\[ x^2 + 2x - 8 = 0\ \Rightarrow\ x = 2,\ -4 \]
\[ x^2 + 2x - 8 = 0 \]
\[ D = 2^2 - 4\cdot1\cdot(-8) = 4 + 32 = 36 \]
\[ \left(x - 2\right)\left(x + 4\right) = 0 \]
\[ x - 2 = 0 \Rightarrow x = 2\qquad x + 4 = 0 \Rightarrow x = -4 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-2 \pm \sqrt{36}}{2} \]
\[ x = 2\quad\text{or}\quad x = -4 \]

The parabola y = x² + 2x - 8

xy
-40
-3-5
-2-8
-1-9
0-8
1-5
20

Questions

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

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

Does x² + 2x - 8 = 0 factor?

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

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

(-1, -9). The parabola opens upwards, so this is the minimum.

What are the sum and product of the roots?

Sum = −b/a = -2, product = c/a = -8.

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