Solve 5x² - 3x - 9 = 0
The solutions of 5x² - 3x - 9 = 0 are x = (3 + 3√21)/10 or x = (3 - 3√21)/10, that is x ≈ 1.6747727 and x ≈ -1.0747727.
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 = -3, 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 ≈ 1.6747727 and x ≈ -1.0747727.
- 7
Check numerically: for x ≈ 1.6747727 the left side is 0 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 5x^2 - 3x - 9 = 0\ \Rightarrow\ x = \frac{3 + 3\sqrt{21}}{10},\ \frac{3 - 3\sqrt{21}}{10} \]
\[ 5x^2 - 3x - 9 = 0 \]
\[ D = (-3)^2 - 4\cdot5\cdot(-9) = 9 + 180 = 189 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{3 \pm \sqrt{189}}{10} \]
\[ x = \frac{3 + 3\sqrt{21}}{10}\quad\text{or}\quad x = \frac{3 - 3\sqrt{21}}{10} \]The parabola y = 5x² - 3x - 9
| x | y |
|---|---|
| -2.7 | 35.55 |
| -1.7 | 10.55 |
| -0.7 | -4.45 |
| 0.3 | -9.45 |
| 1.3 | -4.45 |
| 2.3 | 10.55 |
| 3.3 | 35.55 |
Questions
What are the solutions of 5x² - 3x - 9 = 0?
The solutions of 5x² - 3x - 9 = 0 are x = (3 + 3√21)/10 or x = (3 - 3√21)/10, that is x ≈ 1.6747727 and x ≈ -1.0747727.
Does 5x² - 3x - 9 = 0 factor?
Not over the integers, because the discriminant 189 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 5x² - 3x - 9?
(3/10, -189/20). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = 3/5, product = c/a = -9/5.