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**Linear Equations in Two Variables**

Digital Lesson Linear Equations in Two Variables

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**This is the graph of the equation 2x + 3y = 12.**

Equations of the form ax + by = c are called linear equations in two variables. x y 2 -2 This is the graph of the equation 2x + 3y = 12. (0,4) (6,0) The point (0,4) is the y-intercept. The point (6,0) is the x-intercept. Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Linear Equations

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**The slope of a line is a number, m, which measures its steepness.**

y x 2 -2 m is undefined m = 2 m = 1 2 m = 0 m = - 1 4 Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Slope of a Line

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**The slope of the line passing through the two points (x1, y1) and (x2, y2) is given by the formula**

y2 – y1 x2 – x1 m = , (x1 ≠ x2 ). x y The slope is the change in y divided by the change in x as we move along the line from (x1, y1) to (x2, y2). (x1, y1) (x2, y2) x2 – x1 y2 – y1 change in y change in x Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Slope Formula

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**Example: Find the slope of the line passing through the points (2, 3) and (4, 5).**

Use the slope formula with x1= 2, y1 = 3, x2 = 4, and y2 = 5. y2 – y1 x2 – x1 m = 5 – 3 4 – 2 = = 2 = 1 x y (4, 5) (2, 3) 2 2 Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Example: Find Slope

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**The slope is m and the y-intercept is (0, b).**

A linear equation written in the form y = mx + b is in slope-intercept form. The slope is m and the y-intercept is (0, b). To graph an equation in slope-intercept form: 1. Write the equation in the form y = mx + b. Identify m and b. 2. Plot the y-intercept (0, b). 3. Starting at the y-intercept, find another point on the line using the slope. 4. Draw the line through (0, b) and the point located using the slope. Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Slope-Intercept Form

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**Example: Graph the line y = 2x – 4.**

The equation y = 2x – 4 is in the slope-intercept form. So, m = 2 and b = - 4. x y 2. Plot the y-intercept, (0, - 4). 1 = change in y change in x m = 2 3. The slope is 2. (1, -2) 2 4. Start at the point (0, 4) Count 1 unit to the right and 2 units up to locate a second point on the line. (0, - 4) 1 The point (1, -2) is also on the line. 5. Draw the line through (0, 4) and (1, -2). Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Example: y=mx+b

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**A linear equation written in the form y – y1 = m(x – x1) is in point-slope form.**

The graph of this equation is a line with slope m passing through the point (x1, y1). Example: The graph of the equation y – 3 = - (x – 4) is a line of slope m = - passing through the point (4, 3). 1 2 x y 4 8 m = - 1 2 (4, 3) Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Point-Slope Form

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**Example: Point-Slope Form**

Example: Write the slope-intercept form for the equation of the line through the point (-2, 5) with a slope of 3. Use the point-slope form, y – y1 = m(x – x1), with m = 3 and (x1, y1) = (-2, 5). y – y1 = m(x – x1) Point-slope form y – y1 = 3(x – x1) Let m = 3. y – 5 = 3(x – (-2)) Let (x1, y1) = (-2, 5). y – 5 = 3(x + 2) Simplify. y = 3x + 11 Slope-intercept form Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Example: Point-Slope Form

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**Example: Slope-Intercept Form**

Example: Write the slope-intercept form for the equation of the line through the points (4, 3) and (-2, 5). 2 1 5 – 3 -2 – 4 = - 6 3 Calculate the slope. m = y – y1 = m(x – x1) Point-slope form Use m = - and the point (4, 3). y – 3 = - (x – 4) 1 3 Slope-intercept form y = - x + 13 3 1 Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Example: Slope-Intercept Form

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**Example: Parallel Lines**

Two lines are parallel if they have the same slope. If the lines have slopes m1 and m2, then the lines are parallel whenever m1 = m2. x y y = 2x + 4 (0, 4) Example: The lines y = 2x – 3 and y = 2x + 4 have slopes m1 = 2 and m2 = 2. y = 2x – 3 (0, -3) The lines are parallel. Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Example: Parallel Lines

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**Example: Perpendicular Lines**

Two lines are perpendicular if their slopes are negative reciprocals of each other. If two lines have slopes m1 and m2, then the lines are perpendicular whenever x y 1 m1 m2= - or m1m2 = -1. y = 3x – 1 y = x + 4 1 3 Example: The lines y = 3x – 1 and y = - x + 4 have slopes m1 = 3 and m2 = 1 3 (0, 4) (0, -1) The lines are perpendicular. Copyright © by Brooks/Cole, Cengage Learning. All rights reserved. Example: Perpendicular Lines

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2.4 Writing the Equation of a Line

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