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Solve -2x² + 12x + 8 = 0

The solutions of -2x² + 12x + 8 = 0 are x = 3 + √13 or x = 3 - √13, that is x ≈ -0.60555128 and x ≈ 6.6055513.

x² +x += 0
−2x2+12x+8=0⇒x=3+13, 3−13-2x^2 + 12x + 8 = 0 \Rightarrow x = 3 + \sqrt{13},\ 3 - \sqrt{13}
Discriminant D = 208 · x ≈ -0.605551, 6.60555

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

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Graph of y = -2x² + 12x + 8-22468-50-40-30-20-10102030x = -0.60555128x = 6.6055513vertexy = -2x² + 12x + 8
Graph of y = -2x² + 12x + 8. The curve crosses the x-axis at x = -0.60555128 and x = 6.6055513; the vertex is at (3, 26).

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 = 12, c = 8.

    −2x2+12x+8=0-2x^2 + 12x + 8 = 0
  2. 2

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

    D=122−4⋅(−2)⋅8=144+64=208D = 12^2 - 4\cdot(-2)\cdot8 = 144 + 64 = 208
  3. 3

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

  4. 4

    The quadratic formula works for every quadratic:

    x=−b±D2a=−12±208−4x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-12 \pm \sqrt{208}}{-4}
  5. 5

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

    x=3+13orx=3−13x = 3 + \sqrt{13}\quad\text{or}\quad x = 3 - \sqrt{13}
  6. 6

    As decimals: x ≈ -0.60555128 and x ≈ 6.6055513.

  7. 7

    Check numerically: for x ≈ -0.60555128 the left side is 2.6645353e-15 (zero up to rounding) ✓

LaTeX source of the formulas on this page
\[ -2x^2 + 12x + 8 = 0\ \Rightarrow\ x = 3 + \sqrt{13},\ 3 - \sqrt{13} \]
\[ -2x^2 + 12x + 8 = 0 \]
\[ D = 12^2 - 4\cdot(-2)\cdot8 = 144 + 64 = 208 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-12 \pm \sqrt{208}}{-4} \]
\[ x = 3 + \sqrt{13}\quad\text{or}\quad x = 3 - \sqrt{13} \]

The parabola y = -2x² + 12x + 8

xy
08
118
224
326
424
518
68

Questions

What are the solutions of -2x² + 12x + 8 = 0?

The solutions of -2x² + 12x + 8 = 0 are x = 3 + √13 or x = 3 - √13, that is x ≈ -0.60555128 and x ≈ 6.6055513.

Does -2x² + 12x + 8 = 0 factor?

Not over the integers, because the discriminant 208 is not a perfect square. Use the quadratic formula.

What is the vertex of y = -2x² + 12x + 8?

(3, 26). The parabola opens downwards, so this is the maximum.

What are the sum and product of the roots?

Sum = −b/a = 6, product = c/a = -4.

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    Solve -2x² + 12x + 8 = 0: x = 3 + √13 or x = 3 - √13 (steps) | WordGPT