Solve 5x² - 4x - 8 = 0
The solutions of 5x² - 4x - 8 = 0 are x = (2 + 2√11)/5 or x = (2 - 2√11)/5, that is x ≈ 1.7266499 and x ≈ -0.92664992.
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 = -4, c = -8.
- 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 ≈ 1.7266499 and x ≈ -0.92664992.
- 7
Check numerically: for x ≈ 1.7266499 the left side is 0 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 5x^2 - 4x - 8 = 0\ \Rightarrow\ x = \frac{2 + 2\sqrt{11}}{5},\ \frac{2 - 2\sqrt{11}}{5} \]
\[ 5x^2 - 4x - 8 = 0 \]
\[ D = (-4)^2 - 4\cdot5\cdot(-8) = 16 + 160 = 176 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{4 \pm \sqrt{176}}{10} \]
\[ x = \frac{2 + 2\sqrt{11}}{5}\quad\text{or}\quad x = \frac{2 - 2\sqrt{11}}{5} \]The parabola y = 5x² - 4x - 8
| x | y |
|---|---|
| -2.6 | 36.2 |
| -1.6 | 11.2 |
| -0.6 | -3.8 |
| 0.4 | -8.8 |
| 1.4 | -3.8 |
| 2.4 | 11.2 |
| 3.4 | 36.2 |
Questions
What are the solutions of 5x² - 4x - 8 = 0?
The solutions of 5x² - 4x - 8 = 0 are x = (2 + 2√11)/5 or x = (2 - 2√11)/5, that is x ≈ 1.7266499 and x ≈ -0.92664992.
Does 5x² - 4x - 8 = 0 factor?
Not over the integers, because the discriminant 176 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 5x² - 4x - 8?
(2/5, -44/5). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = 4/5, product = c/a = -8/5.