Solve -3x² - 8x - 1 = 0
The solutions of -3x² - 8x - 1 = 0 are x = (-4 + √13)/3 or x = (-4 - √13)/3, that is x ≈ -2.5351838 and x ≈ -0.13148291.
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 = -3, 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 ≈ -2.5351838 and x ≈ -0.13148291.
- 7
Check numerically: for x ≈ -2.5351838 the left side is 0 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ -3x^2 - 8x - 1 = 0\ \Rightarrow\ x = \frac{-4 + \sqrt{13}}{3},\ \frac{-4 - \sqrt{13}}{3} \]
\[ -3x^2 - 8x - 1 = 0 \]
\[ D = (-8)^2 - 4\cdot(-3)\cdot(-1) = 64 - 12 = 52 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{8 \pm \sqrt{52}}{-6} \]
\[ x = \frac{-4 + \sqrt{13}}{3}\quad\text{or}\quad x = \frac{-4 - \sqrt{13}}{3} \]The parabola y = -3x² - 8x - 1
| x | y |
|---|---|
| -4.3333 | -22.666067 |
| -3.3333 | -7.6662667 |
| -2.3333 | 1.3335333 |
| -1.3333 | 4.3333333 |
| -0.3333 | 1.3331333 |
| 0.6667 | -7.6670667 |
| 1.6667 | -22.667267 |
Questions
What are the solutions of -3x² - 8x - 1 = 0?
The solutions of -3x² - 8x - 1 = 0 are x = (-4 + √13)/3 or x = (-4 - √13)/3, that is x ≈ -2.5351838 and x ≈ -0.13148291.
Does -3x² - 8x - 1 = 0 factor?
Not over the integers, because the discriminant 52 is not a perfect square. Use the quadratic formula.
What is the vertex of y = -3x² - 8x - 1?
(-4/3, 13/3). The parabola opens downwards, so this is the maximum.
What are the sum and product of the roots?
Sum = −b/a = -8/3, product = c/a = 1/3.