Solve 3x² + 12x + 5 = 0
The solutions of 3x² + 12x + 5 = 0 are x = (-6 + √21)/3 or x = (-6 - √21)/3, that is x ≈ -0.47247477 and x ≈ -3.5275252.
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 = 12, 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 ≈ -0.47247477 and x ≈ -3.5275252.
- 7
Check numerically: for x ≈ -0.47247477 the left side is -8.8817842e-16 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ 3x^2 + 12x + 5 = 0\ \Rightarrow\ x = \frac{-6 + \sqrt{21}}{3},\ \frac{-6 - \sqrt{21}}{3} \]
\[ 3x^2 + 12x + 5 = 0 \]
\[ D = 12^2 - 4\cdot3\cdot5 = 144 - 60 = 84 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-12 \pm \sqrt{84}}{6} \]
\[ x = \frac{-6 + \sqrt{21}}{3}\quad\text{or}\quad x = \frac{-6 - \sqrt{21}}{3} \]The parabola y = 3x² + 12x + 5
| x | y |
|---|---|
| -5 | 20 |
| -4 | 5 |
| -3 | -4 |
| -2 | -7 |
| -1 | -4 |
| 0 | 5 |
| 1 | 20 |
Questions
What are the solutions of 3x² + 12x + 5 = 0?
The solutions of 3x² + 12x + 5 = 0 are x = (-6 + √21)/3 or x = (-6 - √21)/3, that is x ≈ -0.47247477 and x ≈ -3.5275252.
Does 3x² + 12x + 5 = 0 factor?
Not over the integers, because the discriminant 84 is not a perfect square. Use the quadratic formula.
What is the vertex of y = 3x² + 12x + 5?
(-2, -7). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = -4, product = c/a = 5/3.