Solve -x² + 4x + 9 = 0
The solutions of -x² + 4x + 9 = 0 are x = 2 + √13 or x = 2 - √13, that is x ≈ -1.6055513 and x ≈ 5.6055513.
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 = -1, b = 4, 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 ≈ -1.6055513 and x ≈ 5.6055513.
- 7
Check numerically: for x ≈ -1.6055513 the left side is 1.7763568e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ -x^2 + 4x + 9 = 0\ \Rightarrow\ x = 2 + \sqrt{13},\ 2 - \sqrt{13} \]
\[ -x^2 + 4x + 9 = 0 \]
\[ D = 4^2 - 4\cdot(-1)\cdot9 = 16 + 36 = 52 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-4 \pm \sqrt{52}}{-2} \]
\[ x = 2 + \sqrt{13}\quad\text{or}\quad x = 2 - \sqrt{13} \]The parabola y = -x² + 4x + 9
| x | y |
|---|---|
| -1 | 4 |
| 0 | 9 |
| 1 | 12 |
| 2 | 13 |
| 3 | 12 |
| 4 | 9 |
| 5 | 4 |
Questions
What are the solutions of -x² + 4x + 9 = 0?
The solutions of -x² + 4x + 9 = 0 are x = 2 + √13 or x = 2 - √13, that is x ≈ -1.6055513 and x ≈ 5.6055513.
Does -x² + 4x + 9 = 0 factor?
Not over the integers, because the discriminant 52 is not a perfect square. Use the quadratic formula.
What is the vertex of y = -x² + 4x + 9?
(2, 13). 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.