Write the equation for the first line and identify the slope and y-intercept.
Example:
y = 4x + 3
m = slope = 4
b = y-intercept = 3
Parallel lines are straight lines that extend to infinity without touching at any point. Perpendicular lines cross each other at a 90-degree angle. Both sets of lines are important for many geometric proofs, so it is important to recognize them graphically and algebraically. You must know the structure of a straight-line equation before you can write equations for parallel or perpendicular lines. The standard form of the equation is "y = mx + b," in which "m" is the slope of the line and "b" is the point where the line crosses the y-axis.
Write the equation for the first line and identify the slope and y-intercept.
Example:
y = 4x + 3
m = slope = 4
b = y-intercept = 3
Copy the first half of the equation for the parallel line. A line is parallel to another if their slopes are identical.
Example:
Original line: y = 4x + 3
Parallel line: y = 4x
Choose a y-intercept different from the original line. Regardless of the magnitude of the new y-intercept, as long as the slope is identical, the two lines will be parallel.
Example:
Original line: y = 4x + 3
Parallel line 1: y = 4x + 7
Parallel line 2: y = 4x - 6
Parallel line 3: y = 4x + 15,328.35
Write the equation for the first line and identify the slope and y-intercept, as with the parallel lines.
Example:
y = 4x + 3
m = slope = 4
b = y-intercept = 3
Transform for the "x" and "y" variable. The angle of rotation is 90 degrees because a perpendicular line intersects the original line at 90 degrees.
Example:
x' = xcos(90) - ysin(90)
y' = xsin(90) + ycos(90)
x' = -y
y' = x
Substitute "y'" and "x'" for "x" and "y" and then write the equation in standard form.
Example:
Original line: y = 4x + 3
Substitute: -x' = 4y' + 3
Standard form: y' = -(1/4)*x - 3/4
The original line, y = 4x + b, is perpendicular to new line, y' = -(1/4)x - 3/4, and any line parallel to the new line, such as y' = -(1/4)x - 10.