Solve -2x² - 5x + 8 = 0
The solutions of -2x² - 5x + 8 = 0 are x = (-5 + √89)/4 or x = (-5 - √89)/4, that is x ≈ -3.6084953 and x ≈ 1.1084953.
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 = -5, 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 ≈ -3.6084953 and x ≈ 1.1084953.
- 7
Check numerically: for x ≈ -3.6084953 the left side is 3.5527137e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ -2x^2 - 5x + 8 = 0\ \Rightarrow\ x = \frac{-5 + \sqrt{89}}{4},\ \frac{-5 - \sqrt{89}}{4} \]
\[ -2x^2 - 5x + 8 = 0 \]
\[ D = (-5)^2 - 4\cdot(-2)\cdot8 = 25 + 64 = 89 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{5 \pm \sqrt{89}}{-4} \]
\[ x = \frac{-5 + \sqrt{89}}{4}\quad\text{or}\quad x = \frac{-5 - \sqrt{89}}{4} \]The parabola y = -2x² - 5x + 8
| x | y |
|---|---|
| -4.25 | -6.875 |
| -3.25 | 3.125 |
| -2.25 | 9.125 |
| -1.25 | 11.125 |
| -0.25 | 9.125 |
| 0.75 | 3.125 |
| 1.75 | -6.875 |
Questions
What are the solutions of -2x² - 5x + 8 = 0?
The solutions of -2x² - 5x + 8 = 0 are x = (-5 + √89)/4 or x = (-5 - √89)/4, that is x ≈ -3.6084953 and x ≈ 1.1084953.
Does -2x² - 5x + 8 = 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 - 5x + 8?
(-5/4, 89/8). The parabola opens downwards, so this is the maximum.
What are the sum and product of the roots?
Sum = −b/a = -5/2, product = c/a = -4.