Solve -3x² - 8x - 12 = 0
-3x² - 8x - 12 = 0 has no real solutions. The two complex solutions are x = (-4 + 2√5 i)/3 and x = (-4 - 2√5 i)/3.
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 = -12.
- 2
Work out the discriminant D = b² − 4ac. Its sign tells you how many real solutions there are.
- 3
D is negative, so there are no real solutions; the two solutions are complex numbers.
- 4
The quadratic formula works for every quadratic:
- 5
Simplify (pull the square out of the root) to get the solutions:
LaTeX source of the formulas on this page
\[ -3x^2 - 8x - 12 = 0\ \Rightarrow\ x = \frac{-4 + 2\sqrt{5}\, i}{3},\ \frac{-4 - 2\sqrt{5}\, i}{3} \]
\[ -3x^2 - 8x - 12 = 0 \]
\[ D = (-8)^2 - 4\cdot(-3)\cdot(-12) = 64 - 144 = -80 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{8 \pm \sqrt{-80}}{-6} \]
\[ x = \frac{-4 + 2\sqrt{5}\, i}{3}\quad\text{or}\quad x = \frac{-4 - 2\sqrt{5}\, i}{3} \]The parabola y = -3x² - 8x - 12
| x | y |
|---|---|
| -4.3333 | -33.666067 |
| -3.3333 | -18.666267 |
| -2.3333 | -9.6664667 |
| -1.3333 | -6.6666667 |
| -0.3333 | -9.6668667 |
| 0.6667 | -18.667067 |
| 1.6667 | -33.667267 |
Questions
What are the solutions of -3x² - 8x - 12 = 0?
-3x² - 8x - 12 = 0 has no real solutions. The two complex solutions are x = (-4 + 2√5 i)/3 and x = (-4 - 2√5 i)/3.
Does -3x² - 8x - 12 = 0 factor?
Not over the integers, because the discriminant -80 is negative. Use the quadratic formula.
What is the vertex of y = -3x² - 8x - 12?
(-4/3, -20/3). 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 = 4.