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Solve x² - 8x + 7 = 0

The solutions of x² - 8x + 7 = 0 are x = 7 or x = 1.

x² +x += 0
x2−8x+7=0⇒x=7, 1x^2 - 8x + 7 = 0 \Rightarrow x = 7,\ 1
Discriminant D = 36 · x ≈ 7, 1

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

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Graph of y = x² - 8x + 72468-10-55101520x = 7x = 1vertexy = x² - 8x + 7
Graph of y = x² - 8x + 7. The curve crosses the x-axis at x = 7 and x = 1; the vertex is at (4, -9).

How to work it out

  1. 1

    Write it in standard form a x² + b x + c = 0 and read off the coefficients: a = 1, b = -8, c = 7.

    x2−8x+7=0x^2 - 8x + 7 = 0
  2. 2

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

    D=(−8)2−4⋅1⋅7=64−28=36D = (-8)^2 - 4\cdot1\cdot7 = 64 - 28 = 36
  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:

    (x−7)(x−1)=0\left(x - 7\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:

    x−7=0⇒x=7x−1=0⇒x=1x - 7 = 0 \Rightarrow x = 7\qquad x - 1 = 0 \Rightarrow x = 1
  6. 6

    Method 2, The quadratic formula works for every quadratic:

    x=−b±D2a=8±362x = \frac{-b \pm \sqrt{D}}{2a} = \frac{8 \pm \sqrt{36}}{2}
  7. 7

    Simplify to get the solutions:

    x=7orx=1x = 7\quad\text{or}\quad x = 1
  8. 8

    Check by putting x = 7 back in: a·x² + b·x + c = 1·(49) + (-8)·(7) + 7 = 0 ✓

LaTeX source of the formulas on this page
\[ x^2 - 8x + 7 = 0\ \Rightarrow\ x = 7,\ 1 \]
\[ x^2 - 8x + 7 = 0 \]
\[ D = (-8)^2 - 4\cdot1\cdot7 = 64 - 28 = 36 \]
\[ \left(x - 7\right)\left(x - 1\right) = 0 \]
\[ x - 7 = 0 \Rightarrow x = 7\qquad x - 1 = 0 \Rightarrow x = 1 \]
\[ x = \frac{-b \pm \sqrt{D}}{2a} = \frac{8 \pm \sqrt{36}}{2} \]
\[ x = 7\quad\text{or}\quad x = 1 \]

The parabola y = x² - 8x + 7

xy
10
2-5
3-8
4-9
5-8
6-5
70

Questions

What are the solutions of x² - 8x + 7 = 0?

The solutions of x² - 8x + 7 = 0 are x = 7 or x = 1.

Does x² - 8x + 7 = 0 factor?

Yes: (x-7)(x-1) = 0.

What is the vertex of y = x² - 8x + 7?

(4, -9). The parabola opens upwards, so this is the minimum.

What are the sum and product of the roots?

Sum = −b/a = 8, product = c/a = 7.

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