Solve -3x² + 7x + 9 = 0
The solutions of -3x² + 7x + 9 = 0 are x = (7 + √157)/6 or x = (7 - √157)/6, that is x ≈ -0.92166068 and x ≈ 3.254994.
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 = 7, c = 9.
- 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 ≈ -0.92166068 and x ≈ 3.254994.
- 7
Check numerically: for x ≈ -0.92166068 the left side is 0 (zero up to rounding) ✓
LaTeX source of the formulas on this page
\[ -3x^2 + 7x + 9 = 0\ \Rightarrow\ x = \frac{7 + \sqrt{157}}{6},\ \frac{7 - \sqrt{157}}{6} \]
\[ -3x^2 + 7x + 9 = 0 \]
\[ D = 7^2 - 4\cdot(-3)\cdot9 = 49 + 108 = 157 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-7 \pm \sqrt{157}}{-6} \]
\[ x = \frac{7 + \sqrt{157}}{6}\quad\text{or}\quad x = \frac{7 - \sqrt{157}}{6} \]The parabola y = -3x² + 7x + 9
| x | y |
|---|---|
| -1.8333 | -13.916067 |
| -0.8333 | 1.0837333 |
| 0.1667 | 10.083533 |
| 1.1667 | 13.083333 |
| 2.1667 | 10.083133 |
| 3.1667 | 1.0829333 |
| 4.1667 | -13.917267 |
Questions
What are the solutions of -3x² + 7x + 9 = 0?
The solutions of -3x² + 7x + 9 = 0 are x = (7 + √157)/6 or x = (7 - √157)/6, that is x ≈ -0.92166068 and x ≈ 3.254994.
Does -3x² + 7x + 9 = 0 factor?
Not over the integers, because the discriminant 157 is not a perfect square. Use the quadratic formula.
What is the vertex of y = -3x² + 7x + 9?
(7/6, 157/12). The parabola opens downwards, so this is the maximum.
What are the sum and product of the roots?
Sum = −b/a = 7/3, product = c/a = -3.