Glossary

3.4 Equations of parallel and perpendicular lines

3 min read•july 22, 2024

Parallel and are key players in geometry. They help us understand relationships between lines and solve real-world problems. Knowing how to find slopes and write equations for these lines is crucial.

We'll learn to identify parallel and perpendicular lines using slopes and equations. This knowledge is super useful for tackling geometry problems and real-life applications in fields like architecture and engineering.

Equations and Properties of Parallel and Perpendicular Lines

Slope from equation or points

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  • Calculate slope using the slope formula m=y2−y1x2−x1m = \frac{y_2 - y_1}{x_2 - x_1} by plugging in the coordinates of two points (x1,y1)(x_1, y_1) and (x2,y2)(x_2, y_2) on the line
  • Identify the slope mm in the of a line's equation [y = mx + b](https://www.fiveableKeyTerm:y_=_mx_+_b) where bb represents the (point where the line crosses the y-axis)
  • Determine the slope mm in the of a line's equation y−y1=m(x−x1)y - y_1 = m(x - x_1) using the coordinates of a single point (x1,y1)(x_1, y_1) on the line
  • Find the slope of a line in general form [Ax + By + C](https://www.fiveableKeyTerm:ax_+_by_+_c) = 0 by rearranging the equation to solve for m=−ABm = -\frac{A}{B}

Slopes of parallel vs perpendicular lines

  • Recognize that have identical slopes meaning if the slope of line 1 equals the slope of line 2 (m1=m2m_1 = m_2) then the lines are parallel and will never intersect
  • Identify perpendicular lines as having slopes that are negative reciprocals of each other satisfying the condition m1⋅m2=−1m_1 \cdot m_2 = -1 so if line 1 has a slope of 23\frac{2}{3} then line 2 will have a slope of −32-\frac{3}{2} for the lines to be perpendicular
  • Understand that perpendicular lines intersect at a 90° angle while parallel lines maintain a constant distance between them without ever intersecting

Equations of parallel and perpendicular lines

  • Write the equation of a line parallel to a given line passing through a specific point (x1,y1)(x_1, y_1) using the slope mm of the given line in slope- form y=m(x−x1)+y1y = m(x - x_1) + y_1 or point-slope form y−y1=m(x−x1)y - y_1 = m(x - x_1)
  • Determine the equation of a line perpendicular to a given line passing through a point (x1,y1)(x_1, y_1) by using the of the given line's slope −1m-\frac{1}{m} in slope-intercept form y=−1m(x−x1)+y1y = -\frac{1}{m}(x - x_1) + y_1 or point-slope form y−y1=−1m(x−x1)y - y_1 = -\frac{1}{m}(x - x_1)
  • Apply the properties of parallel and perpendicular lines to find missing information such as the slope or equation of a line given partial information about the lines and their relationships

Applications in coordinate geometry

  • Compare the slopes of lines to determine if they are parallel (m1=m2m_1 = m_2) or perpendicular (m1⋅m2=−1m_1 \cdot m_2 = -1) and use this information to solve geometric problems
  • Calculate the distance between a point and a line by constructing a perpendicular line from the point to the given line and using the distance formula d=(x2−x1)2+(y2−y1)2d = \sqrt{(x_2-x_1)^2 + (y_2-y_1)^2} between the point and the intersection of the perpendicular and given lines
  • Solve problems involving triangles (right triangles, special triangles), quadrilaterals (parallelograms, rectangles, squares), and other polygons on the coordinate plane using the properties of parallel and perpendicular lines
  • Find the point of intersection between two lines by setting their equations equal to each other and solving the resulting system of equations for the xx and yy coordinates of the point
  • Utilize the concepts of parallel and perpendicular lines in real-world applications such as designing buildings (architecture), constructing roads (engineering), and plotting navigational routes (navigation) to solve practical problems

Key Terms to Review (16)

Ax + by + c: The expression 'ax + by + c' represents the general form of a linear equation in two variables, x and y. This format is crucial for determining the relationships between lines, particularly when analyzing parallel and perpendicular lines. The coefficients 'a' and 'b' relate to the slope and orientation of the line, while 'c' indicates the line's vertical position on the graph.
Cartesian Plane: The Cartesian plane is a two-dimensional coordinate system defined by a horizontal axis (x-axis) and a vertical axis (y-axis), allowing for the representation of points, lines, and shapes in a structured way. It provides a framework for understanding geometric concepts through algebraic equations and relationships, facilitating proofs and calculations involving coordinates and dimensions.
Intercept: An intercept is the point at which a line crosses an axis in a coordinate plane, typically referring to the y-intercept or x-intercept. The y-intercept is where the line intersects the y-axis, represented as the point (0, b) in the equation of a line, while the x-intercept is where it intersects the x-axis, noted as (a, 0). Understanding intercepts is essential for analyzing linear equations and their graphical representations, particularly when discussing relationships between parallel and perpendicular lines.
Negative reciprocal: A negative reciprocal refers to two numbers or slopes that, when multiplied together, equal -1. This concept is crucial in understanding relationships between lines, particularly when dealing with parallel and perpendicular lines. If one line has a slope of 'm', the slope of a line that is perpendicular to it will be '-1/m', demonstrating this negative reciprocal relationship.
Parallel Lines: Parallel lines are straight lines in a plane that never meet, no matter how far they are extended, and they maintain a constant distance apart. This concept is crucial for understanding various geometrical relationships, the properties of angles formed when parallel lines intersect with transversals, and for using coordinate geometry to prove line relationships.
Perpendicular lines: Perpendicular lines are two lines that intersect at a right angle, which is 90 degrees. This relationship creates unique properties in geometry, such as the ability to form right triangles and the significance of slope in coordinate geometry, connecting various concepts in mathematics.
Perpendicular Postulate: The perpendicular postulate states that through a given point not on a line, there is exactly one line perpendicular to the original line. This fundamental principle establishes how lines interact in a plane and serves as a basis for understanding the nature of angles formed by intersecting lines, particularly in the context of parallel and perpendicular lines.
Point-slope form: Point-slope form is a way to express the equation of a line when you know a point on the line and its slope. It is represented as $$y - y_1 = m(x - x_1)$$, where $$m$$ is the slope of the line and $$(x_1, y_1)$$ are the coordinates of the known point. This form is useful for quickly writing equations of lines based on specific points and slopes, which connects to various concepts in geometry and algebra.
Same slope: Same slope refers to the property of two lines that have identical steepness or inclination when graphed on a coordinate plane. This characteristic is crucial when identifying parallel lines, as parallel lines maintain the same slope but never intersect, regardless of their y-intercepts. Understanding this concept aids in solving problems related to the equations of lines and their relationships with one another.
Slope-intercept form: Slope-intercept form is a way to express the equation of a line using the formula $$y = mx + b$$, where $$m$$ represents the slope of the line and $$b$$ represents the y-intercept, or where the line crosses the y-axis. This format makes it easy to identify key characteristics of a line, such as its steepness and its starting point on the y-axis. By using this form, you can quickly graph a line and understand how it behaves in relation to other lines and shapes on a coordinate plane.
Transversal: A transversal is a line that intersects two or more other lines at distinct points. The concept of a transversal is essential for understanding the relationships between angles formed when it crosses parallel or perpendicular lines, which can help establish their properties and prove their relationships.
X-intercept: The x-intercept is the point where a line crosses the x-axis on a graph, which occurs when the value of y is zero. This key feature helps in understanding the behavior of linear equations and their graphical representations, especially in relation to parallel and perpendicular lines, as it provides critical information about the position and slope of the lines involved.
Y = -1/2x + 4 is perpendicular to y = 2x + 5: The equation y = -1/2x + 4 represents a line that is perpendicular to the line represented by y = 2x + 5. Perpendicular lines intersect at a right angle (90 degrees), and their slopes are negative reciprocals of each other. In this case, the slope of the first line is -1/2, and the slope of the second line is 2, which confirms that they are perpendicular because the product of their slopes equals -1.
Y = 2x + 3 is parallel to y = 2x - 1: The equations y = 2x + 3 and y = 2x - 1 represent two lines that are parallel to each other, meaning they have the same slope but different y-intercepts. In this case, both lines have a slope of 2, which indicates they rise two units for every one unit they run horizontally, ensuring they never intersect. This concept is fundamental in understanding the relationship between parallel lines in a coordinate system.
Y = mx + b: The equation y = mx + b represents the slope-intercept form of a linear equation, where 'm' is the slope of the line and 'b' is the y-intercept. This format is essential in understanding the relationships between parallel and perpendicular lines, as it allows for easy identification of slopes and intercepts. By manipulating this equation, one can derive conditions for lines to be parallel or perpendicular based on their slopes.
Y-intercept: The y-intercept is the point where a line crosses the y-axis on a graph, represented by the coordinate (0, b) where b is the value of the y-intercept. It is a crucial aspect in understanding the equations of lines, especially when determining their positions and relationships to one another, such as in cases of parallelism and perpendicularity.
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