Solve x² + 8x - 4 = 0
The solutions of x² + 8x - 4 = 0 are x = -4 + 2√5 or x = -4 - 2√5, that is x ≈ 0.47213595 and x ≈ -8.472136.
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 = 1, b = 8, c = -4.
- 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 ≈ 0.47213595 and x ≈ -8.472136.
- 7
Check numerically: for x ≈ 0.47213595 the left side is 1.7763568e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ x^2 + 8x - 4 = 0\ \Rightarrow\ x = -4 + 2\sqrt{5},\ -4 - 2\sqrt{5} \]
\[ x^2 + 8x - 4 = 0 \]
\[ D = 8^2 - 4\cdot1\cdot(-4) = 64 + 16 = 80 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-8 \pm \sqrt{80}}{2} \]
\[ x = -4 + 2\sqrt{5}\quad\text{or}\quad x = -4 - 2\sqrt{5} \]The parabola y = x² + 8x - 4
| x | y |
|---|---|
| -7 | -11 |
| -6 | -16 |
| -5 | -19 |
| -4 | -20 |
| -3 | -19 |
| -2 | -16 |
| -1 | -11 |
Questions
What are the solutions of x² + 8x - 4 = 0?
The solutions of x² + 8x - 4 = 0 are x = -4 + 2√5 or x = -4 - 2√5, that is x ≈ 0.47213595 and x ≈ -8.472136.
Does x² + 8x - 4 = 0 factor?
Not over the integers, because the discriminant 80 is not a perfect square. Use the quadratic formula.
What is the vertex of y = x² + 8x - 4?
(-4, -20). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = -8, product = c/a = -4.