Solve 4x² - 8x - 9 = 0
The solutions of 4x² - 8x - 9 = 0 are x = (2 + √13)/2 or x = (2 - √13)/2, that is x ≈ 2.8027756 and x ≈ -0.80277564.
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 = -8, 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 ≈ 2.8027756 and x ≈ -0.80277564.
- 7
Check numerically: for x ≈ 2.8027756 the left side is 3.5527137e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 4x^2 - 8x - 9 = 0\ \Rightarrow\ x = \frac{2 + \sqrt{13}}{2},\ \frac{2 - \sqrt{13}}{2} \]
\[ 4x^2 - 8x - 9 = 0 \]
\[ D = (-8)^2 - 4\cdot4\cdot(-9) = 64 + 144 = 208 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{8 \pm \sqrt{208}}{8} \]
\[ x = \frac{2 + \sqrt{13}}{2}\quad\text{or}\quad x = \frac{2 - \sqrt{13}}{2} \]The parabola y = 4x² - 8x - 9
| x | y |
|---|---|
| -2 | 23 |
| -1 | 3 |
| 0 | -9 |
| 1 | -13 |
| 2 | -9 |
| 3 | 3 |
| 4 | 23 |
Questions
What are the solutions of 4x² - 8x - 9 = 0?
The solutions of 4x² - 8x - 9 = 0 are x = (2 + √13)/2 or x = (2 - √13)/2, that is x ≈ 2.8027756 and x ≈ -0.80277564.
Does 4x² - 8x - 9 = 0 factor?
Not over the integers, because the discriminant 208 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 4x² - 8x - 9?
(1, -13). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = 2, product = c/a = -9/4.