Solve 4x² - 2x - 5 = 0
The solutions of 4x² - 2x - 5 = 0 are x = (1 + √21)/4 or x = (1 - √21)/4, that is x ≈ 1.3956439 and x ≈ -0.89564392.
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 = -2, c = -5.
- 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 ≈ 1.3956439 and x ≈ -0.89564392.
- 7
Check numerically: for x ≈ 1.3956439 the left side is 0 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 4x^2 - 2x - 5 = 0\ \Rightarrow\ x = \frac{1 + \sqrt{21}}{4},\ \frac{1 - \sqrt{21}}{4} \]
\[ 4x^2 - 2x - 5 = 0 \]
\[ D = (-2)^2 - 4\cdot4\cdot(-5) = 4 + 80 = 84 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{2 \pm \sqrt{84}}{8} \]
\[ x = \frac{1 + \sqrt{21}}{4}\quad\text{or}\quad x = \frac{1 - \sqrt{21}}{4} \]The parabola y = 4x² - 2x - 5
| x | y |
|---|---|
| -2.75 | 30.75 |
| -1.75 | 10.75 |
| -0.75 | -1.25 |
| 0.25 | -5.25 |
| 1.25 | -1.25 |
| 2.25 | 10.75 |
| 3.25 | 30.75 |
Questions
What are the solutions of 4x² - 2x - 5 = 0?
The solutions of 4x² - 2x - 5 = 0 are x = (1 + √21)/4 or x = (1 - √21)/4, that is x ≈ 1.3956439 and x ≈ -0.89564392.
Does 4x² - 2x - 5 = 0 factor?
Not over the integers, because the discriminant 84 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 4x² - 2x - 5?
(1/4, -21/4). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = 1/2, product = c/a = -5/4.