First, let's see it in action. Here are two points (you can drag them) and the equation of the line through them. Explanations follow. Show ../geometry/images/geom-line-equn.js The PointsWe use Cartesian Coordinates to mark a point on a graph by how far along and how far up it is:
StepsThere are 3 steps to find the Equation of the Straight Line :
Step 1: Find the Slope (or Gradient) from 2 PointsWhat is the slope (or gradient) of this line? We know two points:
The slope is the change in height divided by the change in horizontal distance. Looking at this diagram ... Slope m = change in ychange in x = yA − yBxA − xB In other words, we:
Like this: m = change in y change in x = 4−3 6−2 = 1 4 = 0.25 It doesn't matter which point comes first, it still works out the same. Try swapping the points: m = change in y change in x = 3−4 2−6 = −1 −4 = 0.25 Same answer. Step 2: The "Point-Slope Formula"Now put that slope and one point into the "Point-Slope Formula" Start with the "point-slope" formula (x1 and y1 are the coordinates of a point on the line): y − y1 = m(x − x1) We can choose any point on the line for x1 and y1, so let's just use point (2,3): y − 3 = m(x − 2) We already calculated the slope "m": m = change in ychange in x = 4−36−2 = 14 And we have: y − 3 = 14(x − 2) That is an answer, but we can simplify it further. Step 3: SimplifyStart with:y − 3 = 14(x − 2) Multiply 14 by (x−2):y − 3 = x4 − 24 Add 3 to both sides:y = x4 − 24 + 3 Simplify:y = x4 + 52 And we get: y = x4 + 52 Which is now in the Slope-Intercept (y = mx + b) form. Check It!Let us confirm by testing with the second point (6,4): y = x/4 + 5/2 = 6/4 + 2.5 = 1.5 + 2.5 = 4 Yes, when x=6 then y=4, so it works! Another ExampleExample: What is the equation of this line?Start with the "point-slope" formula: y − y1 = m(x − x1) Put in these values:
And we get: y − 6 = −2(x − 1) Simplify to Slope-Intercept (y = mx + b) form: y − 6 = −2x + 2 y = −2x + 8 DONE! The Big ExceptionThe previous method works nicely except for one particular case: a vertical line: A vertical line's gradient is undefined (because we cannot divide by 0): m = yA − yBxA − xB = 4 − 12 − 2 = 30 = undefined But there is still a way of writing the equation: use x= instead of y=, like this: x = 2 7270, 525, 526, 1165, 1166, 7291, 7292, 7300, 7301, 7302 |