Solve -2x² - 8x + 9 = 0
The solutions of -2x² - 8x + 9 = 0 are x = (-4 + √34)/2 or x = (-4 - √34)/2, that is x ≈ -4.9154759 and x ≈ 0.91547595.
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 = -8, 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 ≈ -4.9154759 and x ≈ 0.91547595.
- 7
Check numerically: for x ≈ -4.9154759 the left side is 0 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ -2x^2 - 8x + 9 = 0\ \Rightarrow\ x = \frac{-4 + \sqrt{34}}{2},\ \frac{-4 - \sqrt{34}}{2} \]
\[ -2x^2 - 8x + 9 = 0 \]
\[ D = (-8)^2 - 4\cdot(-2)\cdot9 = 64 + 72 = 136 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{8 \pm \sqrt{136}}{-4} \]
\[ x = \frac{-4 + \sqrt{34}}{2}\quad\text{or}\quad x = \frac{-4 - \sqrt{34}}{2} \]The parabola y = -2x² - 8x + 9
| x | y |
|---|---|
| -5 | -1 |
| -4 | 9 |
| -3 | 15 |
| -2 | 17 |
| -1 | 15 |
| 0 | 9 |
| 1 | -1 |
Questions
What are the solutions of -2x² - 8x + 9 = 0?
The solutions of -2x² - 8x + 9 = 0 are x = (-4 + √34)/2 or x = (-4 - √34)/2, that is x ≈ -4.9154759 and x ≈ 0.91547595.
Does -2x² - 8x + 9 = 0 factor?
Not over the integers, because the discriminant 136 is not a perfect square. Use the quadratic formula.
What is the vertex of y = -2x² - 8x + 9?
(-2, 17). The parabola opens downwards, so this is the maximum.
What are the sum and product of the roots?
Sum = −b/a = -4, product = c/a = -9/2.