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Solve -3x² - 9x - 5 = 0

The solutions of -3x² - 9x - 5 = 0 are x = (-9 + √21)/6 or x = (-9 - √21)/6, that is x ≈ -2.2637626 and x ≈ -0.73623738.

x² +x += 0
−3x2−9x−5=0⇒x=−9+216, −9−216-3x^2 - 9x - 5 = 0 \Rightarrow x = \frac{-9 + \sqrt{21}}{6},\ \frac{-9 - \sqrt{21}}{6}
Discriminant D = 21 · x ≈ -2.26376, -0.736237

Change a, b or c (whole numbers, a not 0) and the exact solutions follow.

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Graph of y = -3x² - 9x - 5-4-3-2-11-30-25-20-15-10-55x = -2.2637626x = -0.73623738vertexy = -3x² - 9x - 5
Graph of y = -3x² - 9x - 5. The curve crosses the x-axis at x = -2.2637626 and x = -0.73623738; the vertex is at (-1.5, 1.75).

How to work it out

  1. 1

    Write it in standard form a x² + b x + c = 0 and read off the coefficients: a = -3, b = -9, c = -5.

    −3x2−9x−5=0-3x^2 - 9x - 5 = 0
  2. 2

    Work out the discriminant D = b² − 4ac. Its sign tells you how many real solutions there are.

    D=(−9)2−4⋅(−3)⋅(−5)=81−60=21D = (-9)^2 - 4\cdot(-3)\cdot(-5) = 81 - 60 = 21
  3. 3

    D is positive, so there are two different real solutions.

  4. 4

    The quadratic formula works for every quadratic:

    x=−b±D2a=9±21−6x = \frac{-b \pm \sqrt{D}}{2a} = \frac{9 \pm \sqrt{21}}{-6}
  5. 5

    Simplify (pull the square out of the root) to get the solutions:

    x=−9+216orx=−9−216x = \frac{-9 + \sqrt{21}}{6}\quad\text{or}\quad x = \frac{-9 - \sqrt{21}}{6}
  6. 6

    As decimals: x ≈ -2.2637626 and x ≈ -0.73623738.

  7. 7

    Check numerically: for x ≈ -2.2637626 the left side is 1.7763568e-15 (zero up to rounding) ✓

LaTeX source of the formulas on this page
\[ -3x^2 - 9x - 5 = 0\ \Rightarrow\ x = \frac{-9 + \sqrt{21}}{6},\ \frac{-9 - \sqrt{21}}{6} \]
\[ -3x^2 - 9x - 5 = 0 \]
\[ D = (-9)^2 - 4\cdot(-3)\cdot(-5) = 81 - 60 = 21 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{9 \pm \sqrt{21}}{-6} \]
\[ x = \frac{-9 + \sqrt{21}}{6}\quad\text{or}\quad x = \frac{-9 - \sqrt{21}}{6} \]

The parabola y = -3x² - 9x - 5

xy
-4.5-25.25
-3.5-10.25
-2.5-1.25
-1.51.75
-0.5-1.25
0.5-10.25
1.5-25.25

Questions

What are the solutions of -3x² - 9x - 5 = 0?

The solutions of -3x² - 9x - 5 = 0 are x = (-9 + √21)/6 or x = (-9 - √21)/6, that is x ≈ -2.2637626 and x ≈ -0.73623738.

Does -3x² - 9x - 5 = 0 factor?

Not over the integers, because the discriminant 21 is not a perfect square. Use the quadratic formula.

What is the vertex of y = -3x² - 9x - 5?

(-3/2, 7/4). The parabola opens downwards, so this is the maximum.

What are the sum and product of the roots?

Sum = −b/a = -3, product = c/a = 5/3.

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    Solve -3x² - 9x - 5 = 0: x = (-9 + √21)/6 or x = (-9 - √21)/6 (steps) | WordGPT