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Solve -2x² - 7x - 5 = 0

The solutions of -2x² - 7x - 5 = 0 are x = -5/2 or x = -1.

x² +x += 0
−2x2−7x−5=0⇒x=−52, −1-2x^2 - 7x - 5 = 0 \Rightarrow x = -\frac{5}{2},\ -1
Discriminant D = 9 · x ≈ -2.5, -1

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

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Graph of y = -2x² - 7x - 5-5-4-3-2-11-20-15-10-5x = -2.5x = -1vertexy = -2x² - 7x - 5
Graph of y = -2x² - 7x - 5. The curve crosses the x-axis at x = -2.5 and x = -1; the vertex is at (-1.75, 1.125).

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 = -7, c = -5.

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

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

    D=(−7)2−4⋅(−2)⋅(−5)=49−40=9D = (-7)^2 - 4\cdot(-2)\cdot(-5) = 49 - 40 = 9
  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:

    −(2x+5)(x+1)=0-\left(2x + 5\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:

    2x+5=0⇒x=−52x+1=0⇒x=−12x + 5 = 0 \Rightarrow x = -\frac{5}{2}\qquad x + 1 = 0 \Rightarrow x = -1
  6. 6

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=7±9−4x = \frac{-b \pm \sqrt{D}}{2a} = \frac{7 \pm \sqrt{9}}{-4}
  7. 7

    Simplify to get the solutions:

    x=−52orx=−1x = -\frac{5}{2}\quad\text{or}\quad x = -1
  8. 8

    Check by putting x = -5/2 back in: a·x² + b·x + c = -2·(25/4) + (-7)·(-5/2) + (-5) = 0 ✓

LaTeX source of the formulas on this page
\[ -2x^2 - 7x - 5 = 0\ \Rightarrow\ x = -\frac{5}{2},\ -1 \]
\[ -2x^2 - 7x - 5 = 0 \]
\[ D = (-7)^2 - 4\cdot(-2)\cdot(-5) = 49 - 40 = 9 \]
\[ -\left(2x + 5\right)\left(x + 1\right) = 0 \]
\[ 2x + 5 = 0 \Rightarrow x = -\frac{5}{2}\qquad x + 1 = 0 \Rightarrow x = -1 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{7 \pm \sqrt{9}}{-4} \]
\[ x = -\frac{5}{2}\quad\text{or}\quad x = -1 \]

The parabola y = -2x² - 7x - 5

xy
-4.75-16.875
-3.75-6.875
-2.75-0.875
-1.751.125
-0.75-0.875
0.25-6.875
1.25-16.875

Questions

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

The solutions of -2x² - 7x - 5 = 0 are x = -5/2 or x = -1.

Does -2x² - 7x - 5 = 0 factor?

Yes: -1(2x+5)(x+1) = 0.

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

(-7/4, 9/8). The parabola opens downwards, so this is the maximum.

What are the sum and product of the roots?

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

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