Mathematics
Grade 8
15 min
Write a linear equation from a graph
Write a linear equation from a graph
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1
Introduction & Learning Objectives
Learning Objectives
Identify key features of a linear graph, including points and intercepts.
Calculate the slope (rate of change) of a line directly from its graph.
Determine the y-intercept of a line from its graph.
Write the equation of a line in slope-intercept form (y = mx + b) using information from its graph.
Verify the correctness of a linear equation by checking additional points on the graph.
Apply the process of writing linear equations from graphs to solve real-world problems.
Ever wonder how scientists predict the path of a hurricane 🌀 or how much money you'll save each week if you put away a fixed amount?
In this lesson, you'll learn how to translate a visual representation (a graph) into a powerful mathematical statement (a linear equation). This s...
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Key Concepts & Vocabulary
TermDefinitionExample
Linear EquationAn equation whose graph is a straight line. It describes a relationship where one quantity changes at a constant rate with respect to another.`y = 2x + 1` is a linear equation because its graph is a straight line.
GraphA visual representation of data or a relationship between two variables, typically plotted on a coordinate plane.A line drawn on an x-y grid showing how temperature changes with altitude.
Slope (m)A measure of the steepness and direction of a line. It is the ratio of the vertical change (rise) to the horizontal change (run) between any two points on the line.A line that goes up 3 units for every 2 units it goes right has a slope of `3/2`.
Y-intercept (b)The point where a line crosses the y-axis. At this point, the x-coordinate is always...
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Core Formulas
Slope-Intercept Form of a Linear Equation
`y = mx + b`
This formula is used to write the equation of a straight line when you know its slope (`m`) and its y-intercept (`b`).
Slope Formula (from two points)
`m = \frac{y_2 - y_1}{x_2 - x_1}`
This formula calculates the slope (`m`) of a line given any two distinct points `(x_1, y_1)` and `(x_2, y_2)` on the line.
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Challenging
A graph shows the amount of fuel in a car's tank (in gallons) vs. miles driven. The line starts at (0, 15) and goes through (150, 10). What is the equation for the line, and what does the slope represent?
A.y = -1/30x + 15; The car gains 1/30 gallons per mile.
B.y = 30x + 15; The car costs $30 per mile.
C.y = -30x + 15; The car uses 30 gallons per mile.
D.y = -1/30x + 15; The car uses 1/30 gallons per mile.
Challenging
A line is graphed, but the y-intercept is not visible. However, you can clearly see it passes through the points (-5, 13) and (3, -3). What is the equation of this line?
A.y = -2x + 3
B.y = -1/2x + 10.5
C.y = 2x + 23
D.y = -2x - 3
Challenging
A line segment is drawn on a graph with endpoints at (-2, -5) and (4, 7). What is the equation of the infinite line that contains this segment?
A.y = 1/2x - 4
B.y = 6x + 7
C.y = 2x - 1
D.y = -2x + 3
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