Mathematics
Grade 8
15 min
Write a linear equation from a slope and a point
Write a linear equation from a slope and a point
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1
Introduction & Learning Objectives
Learning Objectives
Identify the slope and a given point from a problem description.
Recall and apply the point-slope form of a linear equation.
Substitute given slope and point coordinates into the point-slope form.
Algebraically manipulate the point-slope form into the slope-intercept form ($y = mx + b$).
Write the final linear equation in slope-intercept form.
Verify their derived equation by substituting the given point.
Ever wonder how engineers predict the path of a rocket 🚀 or how economists model sales trends? It all starts with understanding how to describe a straight line!
In this lesson, you'll learn a powerful skill: how to write the equation of a straight line when you know its steepness (slope) and just one point it passes through. This is a fundamental...
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Key Concepts & Vocabulary
TermDefinitionExample
Linear EquationAn equation whose graph is a straight line. It shows a constant rate of change between two variables.$y = 2x + 5$
Slope (m)A measure of the steepness and direction of a line. It represents the 'rise over run' or the change in $y$ divided by the change in $x$.If a line rises 3 units for every 1 unit it moves to the right, its slope is $m = 3$.
Point ($x_1, y_1$)A specific location on a coordinate plane, represented by an ordered pair of numbers.The point $(2, 7)$ means $x=2$ and $y=7$.
Y-intercept (b)The point where a line crosses the y-axis. At this point, the x-coordinate is always 0.In $y = 2x + 5$, the y-intercept is $(0, 5)$, so $b=5$.
Slope-Intercept FormA common way to write linear equations, where $y$ is isolated, making the slope ($m$...
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Core Formulas
Slope-Intercept Form
$y = mx + b$
This form is used to represent a linear equation where 'm' is the slope and 'b' is the y-intercept. Our goal is often to get the equation into this form.
Point-Slope Form
$y - y_1 = m(x - x_1)$
This is the primary formula we use when given a slope 'm' and a specific point '($x_1, y_1$)'. You substitute the given values into this formula, then rearrange it into slope-intercept form.
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Challenging
The equation of a line is y = -4x + 7. Which of the following slope and point combinations could have been used to create this equation?
A.slope = -4, point = (1, 7)
B.slope = 4, point = (1, 3)
C.slope = -4, point = (2, -1)
D.slope = 7, point = (-4, 0)
Challenging
A student was asked to find the equation for a line with slope 2 passing through (3, 1). Their work is shown:
Step 1: y - 1 = 2(x - 3)
Step 2: y - 1 = 2x - 3
Step 3: y = 2x - 2
In which step did they make a mistake?
A.Step 1
B.Step 2
C.Step 3
D.No mistake was made.
Challenging
What is the equation of a line with a slope of -3/4 that passes through the point (-8, -1)?
A.y = (-3/4)x - 7
B.y = (-3/4)x + 5
C.y = (-3/4)x - 6
D.y = (-3/4)x + 7
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