Solve 5x² - 10x - 11 = 0
The solutions of 5x² - 10x - 11 = 0 are x = (5 + 4√5)/5 or x = (5 - 4√5)/5, that is x ≈ 2.7888544 and x ≈ -0.78885438.
Change a, b or c (whole numbers, a not 0) and the exact solutions follow.
How to work it out
- 1
Write it in standard form a x² + b x + c = 0 and read off the coefficients: a = 5, b = -10, c = -11.
- 2
Work out the discriminant D = b² − 4ac. Its sign tells you how many real solutions there are.
- 3
D is positive, so there are two different real solutions.
- 4
The quadratic formula works for every quadratic:
- 5
Simplify (pull the square out of the root) to get the solutions:
- 6
As decimals: x ≈ 2.7888544 and x ≈ -0.78885438.
- 7
Check numerically: for x ≈ 2.7888544 the left side is 0 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 5x^2 - 10x - 11 = 0\ \Rightarrow\ x = \frac{5 + 4\sqrt{5}}{5},\ \frac{5 - 4\sqrt{5}}{5} \]
\[ 5x^2 - 10x - 11 = 0 \]
\[ D = (-10)^2 - 4\cdot5\cdot(-11) = 100 + 220 = 320 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{10 \pm \sqrt{320}}{10} \]
\[ x = \frac{5 + 4\sqrt{5}}{5}\quad\text{or}\quad x = \frac{5 - 4\sqrt{5}}{5} \]The parabola y = 5x² - 10x - 11
| x | y |
|---|---|
| -2 | 29 |
| -1 | 4 |
| 0 | -11 |
| 1 | -16 |
| 2 | -11 |
| 3 | 4 |
| 4 | 29 |
Questions
What are the solutions of 5x² - 10x - 11 = 0?
The solutions of 5x² - 10x - 11 = 0 are x = (5 + 4√5)/5 or x = (5 - 4√5)/5, that is x ≈ 2.7888544 and x ≈ -0.78885438.
Does 5x² - 10x - 11 = 0 factor?
Not over the integers, because the discriminant 320 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 5x² - 10x - 11?
(1, -16). 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 = -11/5.