Solve -2x² + x + 9 = 0
The solutions of -2x² + x + 9 = 0 are x = (1 + √73)/4 or x = (1 - √73)/4, that is x ≈ -1.8860009 and x ≈ 2.3860009.
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 = 1, c = 9.
- 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 ≈ -1.8860009 and x ≈ 2.3860009.
- 7
Check numerically: for x ≈ -1.8860009 the left side is 1.7763568e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ -2x^2 + x + 9 = 0\ \Rightarrow\ x = \frac{1 + \sqrt{73}}{4},\ \frac{1 - \sqrt{73}}{4} \]
\[ -2x^2 + x + 9 = 0 \]
\[ D = 1^2 - 4\cdot(-2)\cdot9 = 1 + 72 = 73 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-1 \pm \sqrt{73}}{-4} \]
\[ x = \frac{1 + \sqrt{73}}{4}\quad\text{or}\quad x = \frac{1 - \sqrt{73}}{4} \]The parabola y = -2x² + x + 9
| x | y |
|---|---|
| -2.75 | -8.875 |
| -1.75 | 1.125 |
| -0.75 | 7.125 |
| 0.25 | 9.125 |
| 1.25 | 7.125 |
| 2.25 | 1.125 |
| 3.25 | -8.875 |
Questions
What are the solutions of -2x² + x + 9 = 0?
The solutions of -2x² + x + 9 = 0 are x = (1 + √73)/4 or x = (1 - √73)/4, that is x ≈ -1.8860009 and x ≈ 2.3860009.
Does -2x² + x + 9 = 0 factor?
Not over the integers, because the discriminant 73 is not a perfect square. Use the quadratic formula.
What is the vertex of y = -2x² + x + 9?
(1/4, 73/8). The parabola opens downwards, so this is the maximum.
What are the sum and product of the roots?
Sum = −b/a = 1/2, product = c/a = -9/2.