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Online calculator: Equation of a line given two points

This online calculator finds the equation of a line given two points on that line, in slope-intercept and parametric forms
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You can find an equation of a straight line given two points laying on that line. However, there exist different forms for a line equation. Here you can find two calculators for an equation of a line:

  • first calculator finds the line equation in slope-intercept form, that is, y=ax+b It also outputs slope and intercept parameters and displays the line on a graph.

  • second calculator finds the line equation in parametric form, that is, x=at+x_0\y=bt+y_0 It also outputs a direction vector and displays line and direction vector on a graph.

Also, the text and formulas below the calculators describe how to find the equation of a line from two points manually.

How to find the equation of a line in slope-intercept form

Let’s find slope-intercept form of a line equation from the two known points (x_0, y_0) and (x_1, y_1). We need to find slope a and intercept b. For two known points we have two equations in respect to a and b y_0=ax_0+b\y_1=ax_1+b

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Let’s subtract the first from the second y_1 - y_0=ax_1 - ax_0+b - b\y_1 - y_0=ax_1 - ax_0\y_1 - y_0=a(x_1 -x_0) And from there a=frac{y_1 - y_0}{x_1 -x_0}

Note that b can be expressed like this b=y-ax So, once we have a, it is easy to calculate b simply by plugging x_0, y_0, a or x_1, y_1, a to the expression above.

Finally, we use the calculated a and b to write the result as y=ax+b

Equation of a vertical line

Note that in the case of a vertical line, the slope and the intercept are undefined because the line runs parallel to the y-axis. The line equation, in this case, becomes x=x_1

Equation of a horizontal line

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Note that in the case of a horizontal line, the slope is zero and the intercept is equal to the y-coordinate of points because the line runs parallel to the x-axis. The line equation, in this case, becomes y=y_1

How to find the slope-intercept equation of a line example

Problem: Find the equation of a line in the slope-intercept form given points (-1, 1) and (2, 4) Solution:

  1. Calculate the slope a: a=frac{y_1 - y_0}{x_1 -x_0} = frac{4 - 1}{2 - (-1)} = frac{3}{3} = 1
  2. Calculate the intercept b using coordinates of either point. Here we use the coordinates (-1, 1): b=y_0 - a x_0 = 1 - 1cdot(-1)=2
  3. Write the final line equation (we omit the slope, because it equals one): y=x+2

And here is how you should enter this problem into the calculator above: slope-intercept line equation example

Parametric line equations

Let’s find out parametric form of a line equation from the two known points (x_0, y_0) and (x_1, y_1). We need to find components of the direction vector also known as displacement vector. D=begin{vmatrix}d_1\d_2end{vmatrix}=begin{vmatrix}x_1-x_0\y_1-y_0end{vmatrix} This vector quantifies the distance and direction of an imaginary motion along a straight line from the first point to the second point.

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Once we have direction vector from x_0, y_0 to x_1, y_1, our parametric equations will be x=d_1t+x_0\y=d_2t+y_0 Note that if t = 0, then x = x_0, y = y_0 and if t = 1, then x = x_1, y = y_1

Equation of a vertical line

Note that in the case of a vertical line, the horizontal displacement is zero because the line runs parallel to the y-axis. The line equations, in this case, become x=x_0\y=d_2t+y_0

Equation of a horizontal line

Note that in the case of a horizontal line, the vertical displacement is zero because the line runs parallel to the x-axis. The line equations, in this case, become x=d_1t+x_0\y=y_0

How to find the parametric equation of a line example

Problem: Find the equation of a line in the parametric form given points (-1, 1) and (2, 4) Solution:

  1. Calculate the displacement vector: D=begin{vmatrix}d_1\d_2end{vmatrix}=begin{vmatrix}x_1-x_0\y_1-y_0end{vmatrix}=begin{vmatrix}2-(-1)\4-1end{vmatrix}=begin{vmatrix}3\3end{vmatrix}
  2. Write the final line equations: x=3t-1\y=3t+1

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