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Solve -2x² - 5x - 7 = 0

-2x² - 5x - 7 = 0 has no real solutions. The two complex solutions are x = (-5 + √31 i)/4 and x = (-5 - √31 i)/4.

x² +x += 0
−2x2−5x−7=0⇒x=−5+31 i4, −5−31 i4 (complex)-2x^2 - 5x - 7 = 0 \Rightarrow x = \frac{-5 + \sqrt{31}\, i}{4},\ \frac{-5 - \sqrt{31}\, i}{4}\ \text{(complex)}
Discriminant D = -31

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

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Graph of y = -2x² - 5x - 7-5-4-3-2-112-35-30-25-20-15-10-5vertexy = -2x² - 5x - 7
Graph of y = -2x² - 5x - 7. The curve never reaches the x-axis, which is why there are no real solutions; the vertex is at (-1.25, -3.875).

How to work it out

  1. 1

    Write it in standard form a x² + b x + c = 0 and read off the coefficients: a = -2, b = -5, c = -7.

    −2x2−5x−7=0-2x^2 - 5x - 7 = 0
  2. 2

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

    D=(−5)2−4⋅(−2)⋅(−7)=25−56=−31D = (-5)^2 - 4\cdot(-2)\cdot(-7) = 25 - 56 = -31
  3. 3

    D is negative, so there are no real solutions; the two solutions are complex numbers.

  4. 4

    The quadratic formula works for every quadratic:

    x=−b±D2a=5±−31−4x = \frac{-b \pm \sqrt{D}}{2a} = \frac{5 \pm \sqrt{-31}}{-4}
  5. 5

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

    x=−5+31 i4orx=−5−31 i4x = \frac{-5 + \sqrt{31}\, i}{4}\quad\text{or}\quad x = \frac{-5 - \sqrt{31}\, i}{4}
LaTeX source of the formulas on this page
\[ -2x^2 - 5x - 7 = 0\ \Rightarrow\ x = \frac{-5 + \sqrt{31}\, i}{4},\ \frac{-5 - \sqrt{31}\, i}{4} \]
\[ -2x^2 - 5x - 7 = 0 \]
\[ D = (-5)^2 - 4\cdot(-2)\cdot(-7) = 25 - 56 = -31 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{5 \pm \sqrt{-31}}{-4} \]
\[ x = \frac{-5 + \sqrt{31}\, i}{4}\quad\text{or}\quad x = \frac{-5 - \sqrt{31}\, i}{4} \]

The parabola y = -2x² - 5x - 7

xy
-4.25-21.875
-3.25-11.875
-2.25-5.875
-1.25-3.875
-0.25-5.875
0.75-11.875
1.75-21.875

Questions

What are the solutions of -2x² - 5x - 7 = 0?

-2x² - 5x - 7 = 0 has no real solutions. The two complex solutions are x = (-5 + √31 i)/4 and x = (-5 - √31 i)/4.

Does -2x² - 5x - 7 = 0 factor?

Not over the integers, because the discriminant -31 is negative. Use the quadratic formula.

What is the vertex of y = -2x² - 5x - 7?

(-5/4, -31/8). The parabola opens downwards, so this is the maximum.

What are the sum and product of the roots?

Sum = −b/a = -5/2, product = c/a = 7/2.

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