The slope of a line measures how steep the line is. http://www.mathopenref.com/coordslope.html You could also say it is the rise over the run; that is, how much the line rises vertically compared with how much it runs horizontally. Being...
Method 1 of 4:
Using a Graph to Find the Slope
Pick two points on the line. Draw dots on the graph to represent these points, and note their coordinates.
Remember when graphing points to list the x-coordinate first, then the y-coordinate.
For example, you might choose the points (-3, -2) and (5, 4).
Determine the rise between the two points. To do this, you must compare the difference in y of the two points. Begin with the first point, the point that is the farthest left on the graph, and count up until you reach the y-coordinate of the second point.
The rise can be positive or negative; that is, you can count up or down to find it.[4] If the line is moving up and to the right, the rise is positive. If the line is moving down and to the right, the rise is negative. [5]
For example, if the y-coordinate of the first point is (-2), and the y-coordinate of the second point is (4), you will count up 6 points, so your rise is 6.
Determine the run between the two points. To do this, you must compare the difference in x of the two points. Begin with the first point, the point that is farthest left on the graph, and count over until you reach the x-coordinate of the second point.
To run is always positive; that is, you can only count from left to right, never right to left. [6]
For example, if the x-coordinate of the first point is (-3), and the x-coordinate of the second point is (5), you will count over 8, so your run is 8.
Make a ratio using the rise over the run to determine the slope. The slope is usually in fraction form, but it can also be a whole number.
For example, if the rise is 6 and the run is 8, then your slope is 68{displaystyle {frac {6}{8}}}Picture 1 of How to Calculate Slope and Intercepts of a Line, which can be simplified to 34{displaystyle {frac {3}{4}}}Picture 2 of How to Calculate Slope and Intercepts of a Line.
Method 2 of 4:
Using Two Given Points to Find the Slope
Set up the formula m=y2−y1x2−x1{displaystyle m={frac {y_{2}-y_{1}}{x_{2}-x_{1}}}}Picture 3 of How to Calculate Slope and Intercepts of a Line. In the formula, m = the slope, (x1,y1){displaystyle (x_{1},y_{1})}Picture 4 of How to Calculate Slope and Intercepts of a Line = the coordinates of the first point, (x2,y2){displaystyle (x_{2},y_{2})}Picture 5 of How to Calculate Slope and Intercepts of a Line = the coordinates of the second point.
Remember that the slope is equal to riserun{displaystyle {frac {rise}{run}}}Picture 6 of How to Calculate Slope and Intercepts of a Line. You are using this formula to find the change in y (rise) over the change in x (run). [7]
Plug the x- and y-coordinates into the formula. Make sure you place the coordinates of the first point ((x1,y1){displaystyle (x_{1},y_{1})}Picture 7 of How to Calculate Slope and Intercepts of a Line) and the second point ((x2,y2){displaystyle (x_{2},y_{2})}Picture 8 of How to Calculate Slope and Intercepts of a Line) in the correct positions in the formula, or else you will not calculate the correct slope.
For example, given the points (-3, -2) and (5, 4), your formula will look like this: m=4−(−2)5−(−3){displaystyle m={frac {4-(-2)}{5-(-3)}}}Picture 9 of How to Calculate Slope and Intercepts of a Line.
Complete the calculation and simplify, if possible. This will give you the slope as a fraction or whole number.
For example, if your slope is m=4−(−2)5−(−3){displaystyle m={frac {4-(-2)}{5-(-3)}}}Picture 10 of How to Calculate Slope and Intercepts of a Lineyou should calculate 4−(−2)=6{displaystyle 4-(-2)=6}Picture 11 of How to Calculate Slope and Intercepts of a Line in the numerator (Remember when subtracting a negative number, you add.) and 5−(−3)=8{displaystyle 5-(-3)=8}Picture 12 of How to Calculate Slope and Intercepts of a Line in the denominator. You can simplify 68{displaystyle {frac {6}{8}}}Picture 13 of How to Calculate Slope and Intercepts of a Line to 34{displaystyle {frac {3}{4}}}Picture 14 of How to Calculate Slope and Intercepts of a Line, so m=34{displaystyle m={frac {3}{4}}}Picture 15 of How to Calculate Slope and Intercepts of a Line.
Method 3 of 4:
Finding the y-intercept, Given the Slope and One Point
Set up the formula y=mx+b{displaystyle y=mx+b}Picture 16 of How to Calculate Slope and Intercepts of a Line. In the formula, y = the y-coordinate of any point on the line, m = slope, x = the x-coordinate of any point on the line, and b = the y-intercept.
y=mx+b{displaystyle y=mx+b}Picture 17 of How to Calculate Slope and Intercepts of a Line is the equation of a line. [8]
The y-intercept is the point at which the line crosses the y-axis.
EXPERT TIP
Picture 18 of How to Calculate Slope and Intercepts of a Line
Grace Imson, MA
Math Instructor, City College of San Francisco
Grace Imson is a math teacher with over 40 years of teaching experience. Grace is currently a math instructor at the City College of San Francisco and was previously in the Math Department at Saint Louis University. She has taught math at the elementary, middle, high school, and college levels. She has an MA in Education, specializing in Administration and Supervision from Saint Louis University.
Picture 19 of How to Calculate Slope and Intercepts of a Line
Grace Imson, MA Math Instructor, City College of San Francisco
Our Expert Agrees: If you have the slope and one point, plug them into the equation of the line. In y = mx + b, m is the slope, and the point coordinate will contain both x and y. Then, solve for b to find the y-intercept.
Plug in the slope and the coordinates of one point in the line. Remember, the slope is equal to the rise over the run. If you need help finding the slope, see the instructions above.
For example, if the slope is 34{displaystyle {frac {3}{4}}}Picture 20 of How to Calculate Slope and Intercepts of a Line, and on point on the line is (5,4), then the formula will look like this: 4=34(5)+b{displaystyle 4={frac {3}{4}}(5)+b}Picture 21 of How to Calculate Slope and Intercepts of a Line.
Complete the equation, solving for b. First multiply the slope and the x-coordinate. Subtract this number from both sides to solve for b.
In the example problem the equation becomes 4=334+b{displaystyle 4=3{frac {3}{4}}+b}Picture 22 of How to Calculate Slope and Intercepts of a Line. Subtracting 334{displaystyle 3{frac {3}{4}}}Picture 23 of How to Calculate Slope and Intercepts of a Line from both sides, you end up with 14=b{displaystyle {frac {1}{4}}=b}Picture 24 of How to Calculate Slope and Intercepts of a Line. So the y-intercept is 14{displaystyle {frac {1}{4}}}Picture 25 of How to Calculate Slope and Intercepts of a Line.
Check your work. On a coordinate graph, plot your known point, then draw a line using the slope. To find the y-intercept, look for the point where the line crosses the y-axis.
For example, if the slope is 34{displaystyle {frac {3}{4}}}Picture 26 of How to Calculate Slope and Intercepts of a Line, and one point is (5,4), draw a point at (5,4), then draw other points along the line by counting to the left 3 and down 4. When you draw a line through the points, you should see the line cross the y-axis just above the (0,0) coordinate.
Method 4 of 4:
Finding the x-intercept, Given the Slope and Y-intercept
Set up the formula y=mx+b{displaystyle y=mx+b}Picture 27 of How to Calculate Slope and Intercepts of a Line. In the formula, y = the y-coordinate of any point on the line, m = slope, x = the x-coordinate of any point on the line, and b = the y-intercept.
y=mx+b{displaystyle y=mx+b}Picture 28 of How to Calculate Slope and Intercepts of a Line is the equation of a line. [9]
The x-intercept is the point at which the line crosses the x-axis.
Plug the slope and y-intercept into the formula. Remember, the slope is equal to the rise over the run. If you need help finding the slope, see the instructions above.
For example, if the slope is 34{displaystyle {frac {3}{4}}}Picture 29 of How to Calculate Slope and Intercepts of a Line, and the y-intercept is 14{displaystyle {frac {1}{4}}}Picture 30 of How to Calculate Slope and Intercepts of a Line, the formula will look like this: y=34x+14{displaystyle y={frac {3}{4}}x+{frac {1}{4}}}Picture 31 of How to Calculate Slope and Intercepts of a Line.
Set y to 0.[10] You are looking for the x-intercept, the point at which the line crosses the x-axis. At this point, the y-coordinate will equal zero. So if we set y to 0, and solve for the corresponding x-coordinate, we will find the point (x, 0), which will be the x-intercept.
In the example problem, the equation becomes 0=34x+14{displaystyle 0={frac {3}{4}}x+{frac {1}{4}}}Picture 32 of How to Calculate Slope and Intercepts of a Line.
Complete the equation, solving for x. First subtract the y-intercept from both sides. Then divide both sides by the slope.
In the example problem the equation becomes −14=34x{displaystyle {frac {-1}{4}}={frac {3}{4}}x}Picture 33 of How to Calculate Slope and Intercepts of a Line. Dividing both sides by 34{displaystyle {frac {3}{4}}}Picture 34 of How to Calculate Slope and Intercepts of a Line, you end up with −412=x{displaystyle {frac {-4}{12}}=x}Picture 35 of How to Calculate Slope and Intercepts of a Line. This simplifies to −13=x{displaystyle {frac {-1}{3}}=x}Picture 36 of How to Calculate Slope and Intercepts of a Line. So the point at which the line crosses the x-axis is (−13,0){displaystyle ({frac {-1}{3}},0)}Picture 37 of How to Calculate Slope and Intercepts of a Line. So the x-intercept is −13{displaystyle {frac {-1}{3}}}Picture 38 of How to Calculate Slope and Intercepts of a Line.
Check your work. On a coordinate graph, plot your y-intercept, then draw a line using the slope. To find the x-intercept, look for the point where the line crosses the x-axis.
For example, if the slope is 34{displaystyle {frac {3}{4}}}Picture 39 of How to Calculate Slope and Intercepts of a Line, and the y-intercept is (0,14){displaystyle (0,{frac {1}{4}})}Picture 40 of How to Calculate Slope and Intercepts of a Line, draw a point at (0,14){displaystyle (0,{frac {1}{4}})}Picture 41 of How to Calculate Slope and Intercepts of a Line, then draw other points along the line by counting to the left 3 and down 4, and to the right 3 and up 4. When you draw a line through the points, you should see the line cross the x-axis just left of the (0,0) coordinate.
Picture 42 of How to Calculate Slope and Intercepts of a Line