Solve 5x² + 8x + 1 = 0
The solutions of 5x² + 8x + 1 = 0 are x = (-4 + √11)/5 or x = (-4 - √11)/5, that is x ≈ -0.13667504 and x ≈ -1.463325.
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 = 8, c = 1.
- 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 ≈ -0.13667504 and x ≈ -1.463325.
- 7
Check numerically: for x ≈ -0.13667504 the left side is -2.220446e-16 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 5x^2 + 8x + 1 = 0\ \Rightarrow\ x = \frac{-4 + \sqrt{11}}{5},\ \frac{-4 - \sqrt{11}}{5} \]
\[ 5x^2 + 8x + 1 = 0 \]
\[ D = 8^2 - 4\cdot5\cdot1 = 64 - 20 = 44 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-8 \pm \sqrt{44}}{10} \]
\[ x = \frac{-4 + \sqrt{11}}{5}\quad\text{or}\quad x = \frac{-4 - \sqrt{11}}{5} \]The parabola y = 5x² + 8x + 1
| x | y |
|---|---|
| -3.8 | 42.8 |
| -2.8 | 17.8 |
| -1.8 | 2.8 |
| -0.8 | -2.2 |
| 0.2 | 2.8 |
| 1.2 | 17.8 |
| 2.2 | 42.8 |
Questions
What are the solutions of 5x² + 8x + 1 = 0?
The solutions of 5x² + 8x + 1 = 0 are x = (-4 + √11)/5 or x = (-4 - √11)/5, that is x ≈ -0.13667504 and x ≈ -1.463325.
Does 5x² + 8x + 1 = 0 factor?
Not over the integers, because the discriminant 44 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 5x² + 8x + 1?
(-4/5, -11/5). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = -8/5, product = c/a = 1/5.