Solve 4x² - x - 8 = 0
The solutions of 4x² - x - 8 = 0 are x = (1 + √129)/8 or x = (1 - √129)/8, that is x ≈ 1.5447271 and x ≈ -1.2947271.
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 = -1, c = -8.
- 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 ≈ 1.5447271 and x ≈ -1.2947271.
- 7
Check numerically: for x ≈ 1.5447271 the left side is 0 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 4x^2 - x - 8 = 0\ \Rightarrow\ x = \frac{1 + \sqrt{129}}{8},\ \frac{1 - \sqrt{129}}{8} \]
\[ 4x^2 - x - 8 = 0 \]
\[ D = (-1)^2 - 4\cdot4\cdot(-8) = 1 + 128 = 129 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{1 \pm \sqrt{129}}{8} \]
\[ x = \frac{1 + \sqrt{129}}{8}\quad\text{or}\quad x = \frac{1 - \sqrt{129}}{8} \]The parabola y = 4x² - x - 8
| x | y |
|---|---|
| -2.875 | 27.9375 |
| -1.875 | 7.9375 |
| -0.875 | -4.0625 |
| 0.125 | -8.0625 |
| 1.125 | -4.0625 |
| 2.125 | 7.9375 |
| 3.125 | 27.9375 |
Questions
What are the solutions of 4x² - x - 8 = 0?
The solutions of 4x² - x - 8 = 0 are x = (1 + √129)/8 or x = (1 - √129)/8, that is x ≈ 1.5447271 and x ≈ -1.2947271.
Does 4x² - x - 8 = 0 factor?
Not over the integers, because the discriminant 129 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 4x² - x - 8?
(1/8, -129/16). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = 1/4, product = c/a = -2.