Solve 3x² - 11x - 12 = 0
The solutions of 3x² - 11x - 12 = 0 are x = (11 + √265)/6 or x = (11 - √265)/6, that is x ≈ 4.5464701 and x ≈ -0.87980343.
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 = -11, c = -12.
- 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 ≈ 4.5464701 and x ≈ -0.87980343.
- 7
Check numerically: for x ≈ 4.5464701 the left side is 0 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 3x^2 - 11x - 12 = 0\ \Rightarrow\ x = \frac{11 + \sqrt{265}}{6},\ \frac{11 - \sqrt{265}}{6} \]
\[ 3x^2 - 11x - 12 = 0 \]
\[ D = (-11)^2 - 4\cdot3\cdot(-12) = 121 + 144 = 265 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{11 \pm \sqrt{265}}{6} \]
\[ x = \frac{11 + \sqrt{265}}{6}\quad\text{or}\quad x = \frac{11 - \sqrt{265}}{6} \]The parabola y = 3x² - 11x - 12
| x | y |
|---|---|
| -1.1667 | 4.9172667 |
| -0.1667 | -10.082933 |
| 0.8333 | -19.083133 |
| 1.8333 | -22.083333 |
| 2.8333 | -19.083533 |
| 3.8333 | -10.083733 |
| 4.8333 | 4.9160667 |
Questions
What are the solutions of 3x² - 11x - 12 = 0?
The solutions of 3x² - 11x - 12 = 0 are x = (11 + √265)/6 or x = (11 - √265)/6, that is x ≈ 4.5464701 and x ≈ -0.87980343.
Does 3x² - 11x - 12 = 0 factor?
Not over the integers, because the discriminant 265 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 3x² - 11x - 12?
(11/6, -265/12). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = 11/3, product = c/a = -4.