Solve 5x² + 8x + 9 = 0
5x² + 8x + 9 = 0 has no real solutions. The two complex solutions are x = (-4 + √29 i)/5 and x = (-4 - √29 i)/5.
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 = 5, 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 negative, so there are no real solutions; the two solutions are complex numbers.
- 4
The quadratic formula works for every quadratic:
- 5
Simplify (pull the square out of the root) to get the solutions:
LaTeX source of the formulas on this page
\[ 5x^2 + 8x + 9 = 0\ \Rightarrow\ x = \frac{-4 + \sqrt{29}\, i}{5},\ \frac{-4 - \sqrt{29}\, i}{5} \]
\[ 5x^2 + 8x + 9 = 0 \]
\[ D = 8^2 - 4\cdot5\cdot9 = 64 - 180 = -116 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-8 \pm \sqrt{-116}}{10} \]
\[ x = \frac{-4 + \sqrt{29}\, i}{5}\quad\text{or}\quad x = \frac{-4 - \sqrt{29}\, i}{5} \]The parabola y = 5x² + 8x + 9
| x | y |
|---|---|
| -3.8 | 50.8 |
| -2.8 | 25.8 |
| -1.8 | 10.8 |
| -0.8 | 5.8 |
| 0.2 | 10.8 |
| 1.2 | 25.8 |
| 2.2 | 50.8 |
Questions
What are the solutions of 5x² + 8x + 9 = 0?
5x² + 8x + 9 = 0 has no real solutions. The two complex solutions are x = (-4 + √29 i)/5 and x = (-4 - √29 i)/5.
Does 5x² + 8x + 9 = 0 factor?
Not over the integers, because the discriminant -116 is negative. Use the quadratic formula.
What is the vertex of y = 5x² + 8x + 9?
(-4/5, 29/5). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = -8/5, product = c/a = 9/5.