Solve -2x² + 9 = 0
The solutions of -2x² + 9 = 0 are x = (3√2)/2 or x = (-3√2)/2, that is x ≈ -2.1213203 and x ≈ 2.1213203.
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 = 0, 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 ≈ -2.1213203 and x ≈ 2.1213203.
- 7
Check numerically: for x ≈ -2.1213203 the left side is 1.7763568e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ -2x^2 + 9 = 0\ \Rightarrow\ x = \frac{3\sqrt{2}}{2},\ \frac{-3\sqrt{2}}{2} \]
\[ -2x^2 + 9 = 0 \]
\[ D = 0^2 - 4\cdot(-2)\cdot9 = 0 + 72 = 72 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{0 \pm \sqrt{72}}{-4} \]
\[ x = \frac{3\sqrt{2}}{2}\quad\text{or}\quad x = \frac{-3\sqrt{2}}{2} \]The parabola y = -2x² + 9
| x | y |
|---|---|
| -3 | -9 |
| -2 | 1 |
| -1 | 7 |
| 0 | 9 |
| 1 | 7 |
| 2 | 1 |
| 3 | -9 |
Questions
What are the solutions of -2x² + 9 = 0?
The solutions of -2x² + 9 = 0 are x = (3√2)/2 or x = (-3√2)/2, that is x ≈ -2.1213203 and x ≈ 2.1213203.
Does -2x² + 9 = 0 factor?
Not over the integers, because the discriminant 72 is not a perfect square. Use the quadratic formula.
What is the vertex of y = -2x² + 9?
(0, 9). The parabola opens downwards, so this is the maximum.
What are the sum and product of the roots?
Sum = −b/a = 0, product = c/a = -9/2.