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Solve -2x² - 10x - 4 = 0

The solutions of -2x² - 10x - 4 = 0 are x = (-5 + √17)/2 or x = (-5 - √17)/2, that is x ≈ -4.5615528 and x ≈ -0.43844719.

x² +x += 0
−2x2−10x−4=0⇒x=−5+172, −5−172-2x^2 - 10x - 4 = 0 \Rightarrow x = \frac{-5 + \sqrt{17}}{2},\ \frac{-5 - \sqrt{17}}{2}
Discriminant D = 68 · x ≈ -4.56155, -0.438447

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

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Graph of y = -2x² - 10x - 4-7-6-5-4-3-2-112-30-20-1010x = -4.5615528x = -0.43844719vertexy = -2x² - 10x - 4
Graph of y = -2x² - 10x - 4. The curve crosses the x-axis at x = -4.5615528 and x = -0.43844719; the vertex is at (-2.5, 8.5).

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 = -10, c = -4.

    −2x2−10x−4=0-2x^2 - 10x - 4 = 0
  2. 2

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

    D=(−10)2−4⋅(−2)⋅(−4)=100−32=68D = (-10)^2 - 4\cdot(-2)\cdot(-4) = 100 - 32 = 68
  3. 3

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

  4. 4

    The quadratic formula works for every quadratic:

    x=−b±D2a=10±68−4x = \frac{-b \pm \sqrt{D}}{2a} = \frac{10 \pm \sqrt{68}}{-4}
  5. 5

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

    x=−5+172orx=−5−172x = \frac{-5 + \sqrt{17}}{2}\quad\text{or}\quad x = \frac{-5 - \sqrt{17}}{2}
  6. 6

    As decimals: x ≈ -4.5615528 and x ≈ -0.43844719.

  7. 7

    Check numerically: for x ≈ -4.5615528 the left side is -7.1054274e-15 (zero up to rounding) ✓

LaTeX source of the formulas on this page
\[ -2x^2 - 10x - 4 = 0\ \Rightarrow\ x = \frac{-5 + \sqrt{17}}{2},\ \frac{-5 - \sqrt{17}}{2} \]
\[ -2x^2 - 10x - 4 = 0 \]
\[ D = (-10)^2 - 4\cdot(-2)\cdot(-4) = 100 - 32 = 68 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{10 \pm \sqrt{68}}{-4} \]
\[ x = \frac{-5 + \sqrt{17}}{2}\quad\text{or}\quad x = \frac{-5 - \sqrt{17}}{2} \]

The parabola y = -2x² - 10x - 4

xy
-5.5-9.5
-4.50.5
-3.56.5
-2.58.5
-1.56.5
-0.50.5
0.5-9.5

Questions

What are the solutions of -2x² - 10x - 4 = 0?

The solutions of -2x² - 10x - 4 = 0 are x = (-5 + √17)/2 or x = (-5 - √17)/2, that is x ≈ -4.5615528 and x ≈ -0.43844719.

Does -2x² - 10x - 4 = 0 factor?

Not over the integers, because the discriminant 68 is not a perfect square. Use the quadratic formula.

What is the vertex of y = -2x² - 10x - 4?

(-5/2, 17/2). The parabola opens downwards, so this is the maximum.

What are the sum and product of the roots?

Sum = −b/a = -5, product = c/a = 2.

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