Solve 5x² - 9x - 8 = 0
The solutions of 5x² - 9x - 8 = 0 are x = (9 + √241)/10 or x = (9 - √241)/10, that is x ≈ 2.4524175 and x ≈ -0.65241747.
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 = -9, 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 ≈ 2.4524175 and x ≈ -0.65241747.
- 7
Check numerically: for x ≈ 2.4524175 the left side is 3.5527137e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 5x^2 - 9x - 8 = 0\ \Rightarrow\ x = \frac{9 + \sqrt{241}}{10},\ \frac{9 - \sqrt{241}}{10} \]
\[ 5x^2 - 9x - 8 = 0 \]
\[ D = (-9)^2 - 4\cdot5\cdot(-8) = 81 + 160 = 241 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{9 \pm \sqrt{241}}{10} \]
\[ x = \frac{9 + \sqrt{241}}{10}\quad\text{or}\quad x = \frac{9 - \sqrt{241}}{10} \]The parabola y = 5x² - 9x - 8
| x | y |
|---|---|
| -2.1 | 32.95 |
| -1.1 | 7.95 |
| -0.1 | -7.05 |
| 0.9 | -12.05 |
| 1.9 | -7.05 |
| 2.9 | 7.95 |
| 3.9 | 32.95 |
Questions
What are the solutions of 5x² - 9x - 8 = 0?
The solutions of 5x² - 9x - 8 = 0 are x = (9 + √241)/10 or x = (9 - √241)/10, that is x ≈ 2.4524175 and x ≈ -0.65241747.
Does 5x² - 9x - 8 = 0 factor?
Not over the integers, because the discriminant 241 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 5x² - 9x - 8?
(9/10, -241/20). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = 9/5, product = c/a = -8/5.