Solve -3x² + 8x + 8 = 0
The solutions of -3x² + 8x + 8 = 0 are x = (4 + 2√10)/3 or x = (4 - 2√10)/3, that is x ≈ -0.77485177 and x ≈ 3.4415184.
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 = 8.
- 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 ≈ -0.77485177 and x ≈ 3.4415184.
- 7
Check numerically: for x ≈ -0.77485177 the left side is -1.7763568e-15 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ -3x^2 + 8x + 8 = 0\ \Rightarrow\ x = \frac{4 + 2\sqrt{10}}{3},\ \frac{4 - 2\sqrt{10}}{3} \]
\[ -3x^2 + 8x + 8 = 0 \]
\[ D = 8^2 - 4\cdot(-3)\cdot8 = 64 + 96 = 160 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-8 \pm \sqrt{160}}{-6} \]
\[ x = \frac{4 + 2\sqrt{10}}{3}\quad\text{or}\quad x = \frac{4 - 2\sqrt{10}}{3} \]The parabola y = -3x² + 8x + 8
| x | y |
|---|---|
| -1.6667 | -13.667267 |
| -0.6667 | 1.3329333 |
| 0.3333 | 10.333133 |
| 1.3333 | 13.333333 |
| 2.3333 | 10.333533 |
| 3.3333 | 1.3337333 |
| 4.3333 | -13.666067 |
Questions
What are the solutions of -3x² + 8x + 8 = 0?
The solutions of -3x² + 8x + 8 = 0 are x = (4 + 2√10)/3 or x = (4 - 2√10)/3, that is x ≈ -0.77485177 and x ≈ 3.4415184.
Does -3x² + 8x + 8 = 0 factor?
Not over the integers, because the discriminant 160 is not a perfect square. Use the quadratic formula.
What is the vertex of y = -3x² + 8x + 8?
(4/3, 40/3). 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 = -8/3.