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Solve x² - 5x - 10 = 0

The solutions of x² - 5x - 10 = 0 are x = (5 + √65)/2 or x = (5 - √65)/2, that is x ≈ 6.5311289 and x ≈ -1.5311289.

x² +x += 0
x2−5x−10=0⇒x=5+652, 5−652x^2 - 5x - 10 = 0 \Rightarrow x = \frac{5 + \sqrt{65}}{2},\ \frac{5 - \sqrt{65}}{2}
Discriminant D = 65 · x ≈ 6.53113, -1.53113

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

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Graph of y = x² - 5x - 10-4-22468-20-101020x = 6.5311289x = -1.5311289vertexy = x² - 5x - 10
Graph of y = x² - 5x - 10. The curve crosses the x-axis at x = 6.5311289 and x = -1.5311289; the vertex is at (2.5, -16.25).

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 = -5, c = -10.

    x2−5x−10=0x^2 - 5x - 10 = 0
  2. 2

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

    D=(−5)2−4⋅1⋅(−10)=25+40=65D = (-5)^2 - 4\cdot1\cdot(-10) = 25 + 40 = 65
  3. 3

    D is positive, so there are two different real solutions.

  4. 4

    The quadratic formula works for every quadratic:

    x=−b±D2a=5±652x = \frac{-b \pm \sqrt{D}}{2a} = \frac{5 \pm \sqrt{65}}{2}
  5. 5

    Simplify (pull the square out of the root) to get the solutions:

    x=5+652orx=5−652x = \frac{5 + \sqrt{65}}{2}\quad\text{or}\quad x = \frac{5 - \sqrt{65}}{2}
  6. 6

    As decimals: x ≈ 6.5311289 and x ≈ -1.5311289.

  7. 7

    Check numerically: for x ≈ 6.5311289 the left side is 0 (zero up to rounding) ✓

LaTeX source of the formulas on this page
\[ x^2 - 5x - 10 = 0\ \Rightarrow\ x = \frac{5 + \sqrt{65}}{2},\ \frac{5 - \sqrt{65}}{2} \]
\[ x^2 - 5x - 10 = 0 \]
\[ D = (-5)^2 - 4\cdot1\cdot(-10) = 25 + 40 = 65 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{5 \pm \sqrt{65}}{2} \]
\[ x = \frac{5 + \sqrt{65}}{2}\quad\text{or}\quad x = \frac{5 - \sqrt{65}}{2} \]

The parabola y = x² - 5x - 10

xy
-0.5-7.25
0.5-12.25
1.5-15.25
2.5-16.25
3.5-15.25
4.5-12.25
5.5-7.25

Questions

What are the solutions of x² - 5x - 10 = 0?

The solutions of x² - 5x - 10 = 0 are x = (5 + √65)/2 or x = (5 - √65)/2, that is x ≈ 6.5311289 and x ≈ -1.5311289.

Does x² - 5x - 10 = 0 factor?

Not over the integers, because the discriminant 65 is not a perfect square. Use the quadratic formula.

What is the vertex of y = x² - 5x - 10?

(5/2, -65/4). The parabola opens upwards, so this is the minimum.

What are the sum and product of the roots?

Sum = −b/a = 5, product = c/a = -10.

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