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Solve x² - 4 = 0

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

x² +x += 0
x2−4=0⇒x=2, −2x^2 - 4 = 0 \Rightarrow x = 2,\ -2
Discriminant D = 16 · x ≈ 2, -2

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

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Graph of y = x² - 4-4-3-2-11234-551015x = 2x = -2vertexy = x² - 4
Graph of y = x² - 4. The curve crosses the x-axis at x = 2 and x = -2; the vertex is at (0, -4).

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 = 0, c = -4.

    x2−4=0x^2 - 4 = 0
  2. 2

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

    D=02−4⋅1⋅(−4)=0+16=16D = 0^2 - 4\cdot1\cdot(-4) = 0 + 16 = 16
  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+2)=0\left(x - 2\right)\left(x + 2\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+2=0⇒x=−2x - 2 = 0 \Rightarrow x = 2\qquad x + 2 = 0 \Rightarrow x = -2
  6. 6

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=0±162x = \frac{-b \pm \sqrt{D}}{2a} = \frac{0 \pm \sqrt{16}}{2}
  7. 7

    Simplify to get the solutions:

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

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

LaTeX source of the formulas on this page
\[ x^2 - 4 = 0\ \Rightarrow\ x = 2,\ -2 \]
\[ x^2 - 4 = 0 \]
\[ D = 0^2 - 4\cdot1\cdot(-4) = 0 + 16 = 16 \]
\[ \left(x - 2\right)\left(x + 2\right) = 0 \]
\[ x - 2 = 0 \Rightarrow x = 2\qquad x + 2 = 0 \Rightarrow x = -2 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{0 \pm \sqrt{16}}{2} \]
\[ x = 2\quad\text{or}\quad x = -2 \]

The parabola y = x² - 4

xy
-35
-20
-1-3
0-4
1-3
20
35

Questions

What are the solutions of x² - 4 = 0?

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

Does x² - 4 = 0 factor?

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

What is the vertex of y = x² - 4?

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

What are the sum and product of the roots?

Sum = −b/a = 0, product = c/a = -4.

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