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Solve -3x² - 5x + 8 = 0

The solutions of -3x² - 5x + 8 = 0 are x = -8/3 or x = 1.

x² +x += 0
−3x2−5x+8=0⇒x=−83, 1-3x^2 - 5x + 8 = 0 \Rightarrow x = -\frac{8}{3},\ 1
Discriminant D = 121 · x ≈ -2.66667, 1

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

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Graph of y = -3x² - 5x + 8-5-4-3-2-1123-50-40-30-20-1010x = -2.6666667x = 1vertexy = -3x² - 5x + 8
Graph of y = -3x² - 5x + 8. The curve crosses the x-axis at x = -2.6666667 and x = 1; the vertex is at (-0.83333333, 10.083333).

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 = -5, c = 8.

    −3x2−5x+8=0-3x^2 - 5x + 8 = 0
  2. 2

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

    D=(−5)2−4⋅(−3)⋅8=25+96=121D = (-5)^2 - 4\cdot(-3)\cdot8 = 25 + 96 = 121
  3. 3

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

  4. 4

    Method 1, factoring. The roots are rational, so the left side factors:

    −(3x+8)(x−1)=0-\left(3x + 8\right)\left(x - 1\right) = 0
  5. 5

    A product is zero when one of its factors is zero, so set each factor equal to 0 and solve:

    3x+8=0⇒x=−83x−1=0⇒x=13x + 8 = 0 \Rightarrow x = -\frac{8}{3}\qquad x - 1 = 0 \Rightarrow x = 1
  6. 6

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=5±121−6x = \frac{-b \pm \sqrt{D}}{2a} = \frac{5 \pm \sqrt{121}}{-6}
  7. 7

    Simplify to get the solutions:

    x=−83orx=1x = -\frac{8}{3}\quad\text{or}\quad x = 1
  8. 8

    Check by putting x = -8/3 back in: a·x² + b·x + c = -3·(64/9) + (-5)·(-8/3) + 8 = 0 ✓

LaTeX source of the formulas on this page
\[ -3x^2 - 5x + 8 = 0\ \Rightarrow\ x = -\frac{8}{3},\ 1 \]
\[ -3x^2 - 5x + 8 = 0 \]
\[ D = (-5)^2 - 4\cdot(-3)\cdot8 = 25 + 96 = 121 \]
\[ -\left(3x + 8\right)\left(x - 1\right) = 0 \]
\[ 3x + 8 = 0 \Rightarrow x = -\frac{8}{3}\qquad x - 1 = 0 \Rightarrow x = 1 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{5 \pm \sqrt{121}}{-6} \]
\[ x = -\frac{8}{3}\quad\text{or}\quad x = 1 \]

The parabola y = -3x² - 5x + 8

xy
-3.8333-16.916067
-2.8333-1.9162667
-1.83337.0835333
-0.833310.083333
0.16677.0831333
1.1667-1.9170667
2.1667-16.917267

Questions

What are the solutions of -3x² - 5x + 8 = 0?

The solutions of -3x² - 5x + 8 = 0 are x = -8/3 or x = 1.

Does -3x² - 5x + 8 = 0 factor?

Yes: -1(3x+8)(x-1) = 0.

What is the vertex of y = -3x² - 5x + 8?

(-5/6, 121/12). The parabola opens downwards, so this is the maximum.

What are the sum and product of the roots?

Sum = −b/a = -5/3, product = c/a = -8/3.

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