Solve -3x² - 8x + 5 = 0
The solutions of -3x² - 8x + 5 = 0 are x = (-4 + √31)/3 or x = (-4 - √31)/3, that is x ≈ -3.1892548 and x ≈ 0.52258812.
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 = 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 ≈ -3.1892548 and x ≈ 0.52258812.
- 7
Check numerically: for x ≈ -3.1892548 the left side is -3.5527137e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ -3x^2 - 8x + 5 = 0\ \Rightarrow\ x = \frac{-4 + \sqrt{31}}{3},\ \frac{-4 - \sqrt{31}}{3} \]
\[ -3x^2 - 8x + 5 = 0 \]
\[ D = (-8)^2 - 4\cdot(-3)\cdot5 = 64 + 60 = 124 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{8 \pm \sqrt{124}}{-6} \]
\[ x = \frac{-4 + \sqrt{31}}{3}\quad\text{or}\quad x = \frac{-4 - \sqrt{31}}{3} \]The parabola y = -3x² - 8x + 5
| x | y |
|---|---|
| -4.3333 | -16.666067 |
| -3.3333 | -1.6662667 |
| -2.3333 | 7.3335333 |
| -1.3333 | 10.333333 |
| -0.3333 | 7.3331333 |
| 0.6667 | -1.6670667 |
| 1.6667 | -16.667267 |
Questions
What are the solutions of -3x² - 8x + 5 = 0?
The solutions of -3x² - 8x + 5 = 0 are x = (-4 + √31)/3 or x = (-4 - √31)/3, that is x ≈ -3.1892548 and x ≈ 0.52258812.
Does -3x² - 8x + 5 = 0 factor?
Not over the integers, because the discriminant 124 is not a perfect square. Use the quadratic formula.
What is the vertex of y = -3x² - 8x + 5?
(-4/3, 31/3). 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 = -5/3.