Solve 2x² - 9x - 1 = 0
The solutions of 2x² - 9x - 1 = 0 are x = (9 + √89)/4 or x = (9 - √89)/4, that is x ≈ 4.6084953 and x ≈ -0.10849528.
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 = 2, b = -9, c = -1.
- 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 ≈ 4.6084953 and x ≈ -0.10849528.
- 7
Check numerically: for x ≈ 4.6084953 the left side is -7.1054274e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 2x^2 - 9x - 1 = 0\ \Rightarrow\ x = \frac{9 + \sqrt{89}}{4},\ \frac{9 - \sqrt{89}}{4} \]
\[ 2x^2 - 9x - 1 = 0 \]
\[ D = (-9)^2 - 4\cdot2\cdot(-1) = 81 + 8 = 89 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{9 \pm \sqrt{89}}{4} \]
\[ x = \frac{9 + \sqrt{89}}{4}\quad\text{or}\quad x = \frac{9 - \sqrt{89}}{4} \]The parabola y = 2x² - 9x - 1
| x | y |
|---|---|
| -0.75 | 6.875 |
| 0.25 | -3.125 |
| 1.25 | -9.125 |
| 2.25 | -11.125 |
| 3.25 | -9.125 |
| 4.25 | -3.125 |
| 5.25 | 6.875 |
Questions
What are the solutions of 2x² - 9x - 1 = 0?
The solutions of 2x² - 9x - 1 = 0 are x = (9 + √89)/4 or x = (9 - √89)/4, that is x ≈ 4.6084953 and x ≈ -0.10849528.
Does 2x² - 9x - 1 = 0 factor?
Not over the integers, because the discriminant 89 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 2x² - 9x - 1?
(9/4, -89/8). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = 9/2, product = c/a = -1/2.