Solve 3x² + 8x + 7 = 0
3x² + 8x + 7 = 0 has no real solutions. The two complex solutions are x = (-4 + √5 i)/3 and x = (-4 - √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 = 7.
- 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
\[ 3x^2 + 8x + 7 = 0\ \Rightarrow\ x = \frac{-4 + \sqrt{5}\, i}{3},\ \frac{-4 - \sqrt{5}\, i}{3} \]
\[ 3x^2 + 8x + 7 = 0 \]
\[ D = 8^2 - 4\cdot3\cdot7 = 64 - 84 = -20 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-8 \pm \sqrt{-20}}{6} \]
\[ x = \frac{-4 + \sqrt{5}\, i}{3}\quad\text{or}\quad x = \frac{-4 - \sqrt{5}\, i}{3} \]The parabola y = 3x² + 8x + 7
| x | y |
|---|---|
| -4.3333 | 28.666067 |
| -3.3333 | 13.666267 |
| -2.3333 | 4.6664667 |
| -1.3333 | 1.6666667 |
| -0.3333 | 4.6668667 |
| 0.6667 | 13.667067 |
| 1.6667 | 28.667267 |
Questions
What are the solutions of 3x² + 8x + 7 = 0?
3x² + 8x + 7 = 0 has no real solutions. The two complex solutions are x = (-4 + √5 i)/3 and x = (-4 - √5 i)/3.
Does 3x² + 8x + 7 = 0 factor?
Not over the integers, because the discriminant -20 is negative. Use the quadratic formula.
What is the vertex of y = 3x² + 8x + 7?
(-4/3, 5/3). The parabola opens upwards, so this is the minimum.
What are the sum and product of the roots?
Sum = −b/a = -8/3, product = c/a = 7/3.