Solve 4x² - 6x - 9 = 0
The solutions of 4x² - 6x - 9 = 0 are x = (3 + 3√5)/4 or x = (3 - 3√5)/4, that is x ≈ 2.427051 and x ≈ -0.92705098.
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 = 4, b = -6, 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 ≈ 2.427051 and x ≈ -0.92705098.
- 7
Check numerically: for x ≈ 2.427051 the left side is 1.7763568e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 4x^2 - 6x - 9 = 0\ \Rightarrow\ x = \frac{3 + 3\sqrt{5}}{4},\ \frac{3 - 3\sqrt{5}}{4} \]
\[ 4x^2 - 6x - 9 = 0 \]
\[ D = (-6)^2 - 4\cdot4\cdot(-9) = 36 + 144 = 180 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{6 \pm \sqrt{180}}{8} \]
\[ x = \frac{3 + 3\sqrt{5}}{4}\quad\text{or}\quad x = \frac{3 - 3\sqrt{5}}{4} \]The parabola y = 4x² - 6x - 9
| x | y |
|---|---|
| -2.25 | 24.75 |
| -1.25 | 4.75 |
| -0.25 | -7.25 |
| 0.75 | -11.25 |
| 1.75 | -7.25 |
| 2.75 | 4.75 |
| 3.75 | 24.75 |
Questions
What are the solutions of 4x² - 6x - 9 = 0?
The solutions of 4x² - 6x - 9 = 0 are x = (3 + 3√5)/4 or x = (3 - 3√5)/4, that is x ≈ 2.427051 and x ≈ -0.92705098.
Does 4x² - 6x - 9 = 0 factor?
Not over the integers, because the discriminant 180 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 4x² - 6x - 9?
(3/4, -45/4). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = 3/2, product = c/a = -9/4.