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Solve 5x² + 9x + 4 = 0

The solutions of 5x² + 9x + 4 = 0 are x = -4/5 or x = -1.

x² +x += 0
5x2+9x+4=0⇒x=−45, −15x^2 + 9x + 4 = 0 \Rightarrow x = -\frac{4}{5},\ -1
Discriminant D = 1 · x ≈ -0.8, -1

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

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Graph of y = 5x² + 9x + 4-3.5-3-2.5-2-1.5-1-0.50.511.55101520253035x = -0.8x = -1vertexy = 5x² + 9x + 4
Graph of y = 5x² + 9x + 4. The curve crosses the x-axis at x = -0.8 and x = -1; the vertex is at (-0.9, -0.05).

How to work it out

  1. 1

    Write it in standard form a x² + b x + c = 0 and read off the coefficients: a = 5, b = 9, c = 4.

    5x2+9x+4=05x^2 + 9x + 4 = 0
  2. 2

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

    D=92−4⋅5⋅4=81−80=1D = 9^2 - 4\cdot5\cdot4 = 81 - 80 = 1
  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:

    (5x+4)(x+1)=0\left(5x + 4\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:

    5x+4=0⇒x=−45x+1=0⇒x=−15x + 4 = 0 \Rightarrow x = -\frac{4}{5}\qquad x + 1 = 0 \Rightarrow x = -1
  6. 6

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=−9±110x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-9 \pm \sqrt{1}}{10}
  7. 7

    Simplify to get the solutions:

    x=−45orx=−1x = -\frac{4}{5}\quad\text{or}\quad x = -1
  8. 8

    Check by putting x = -4/5 back in: a·x² + b·x + c = 5·(16/25) + 9·(-4/5) + 4 = 0 ✓

LaTeX source of the formulas on this page
\[ 5x^2 + 9x + 4 = 0\ \Rightarrow\ x = -\frac{4}{5},\ -1 \]
\[ 5x^2 + 9x + 4 = 0 \]
\[ D = 9^2 - 4\cdot5\cdot4 = 81 - 80 = 1 \]
\[ \left(5x + 4\right)\left(x + 1\right) = 0 \]
\[ 5x + 4 = 0 \Rightarrow x = -\frac{4}{5}\qquad x + 1 = 0 \Rightarrow x = -1 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{-9 \pm \sqrt{1}}{10} \]
\[ x = -\frac{4}{5}\quad\text{or}\quad x = -1 \]

The parabola y = 5x² + 9x + 4

xy
-3.944.95
-2.919.95
-1.94.95
-0.9-0.05
0.14.95
1.119.95
2.144.95

Questions

What are the solutions of 5x² + 9x + 4 = 0?

The solutions of 5x² + 9x + 4 = 0 are x = -4/5 or x = -1.

Does 5x² + 9x + 4 = 0 factor?

Yes: (5x+4)(x+1) = 0.

What is the vertex of y = 5x² + 9x + 4?

(-9/10, -1/20). The parabola opens upwards, so this is the minimum.

What are the sum and product of the roots?

Sum = −b/a = -9/5, product = c/a = 4/5.

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