1. 1 The graph of y = 2x + 3 is shown. You can see that the line’s y -intercept is 3, and the line’s slope m is 2 : m =2 = rise run = 2121 Slope-Intercept Form 8.5 LESSON Notice that the slope is equal to the coefficient of x in the equation y = 2x + 3. Also notice that the y -intercept is equal to the constant term in the equation. These results are always true for an equation written in slope-intercept form.

2. 2 Slope-Intercept Form Words AlgebraNumbers A linear equation of the form y = mx + b is said to be in slope-intercept form. The slope is m and the y -intercept is b. y = mx + by = 2x + 3 Slope-Intercept Form 8.5 LESSON

3. 3 Identify the slope and y -intercept of the line with the given equation. ANSWER Identifying the Slope and y -Intercept EXAMPLE 1 3x + 5y = 10 5y = –3x + 10 Write original equation. Subtract 3x from each side. 3x + 5y = 10 Write the equation 3x + 5y = 10 in slope-intercept form by solving for y. y =– x + 2 3535 Multiply each side by. 1515 The line has a slope of – and a y -intercept of 2. 3535 SOLUTION Slope-Intercept Form 8.5 LESSON

4. 4 EXAMPLE 2 Graphing an Equation in Slope-Intercept Form Graph the equation y = – x + 4. 2323 1 The y -intercept is 4, so plot the point (0, 4). 2 The slope is – =. 2323 –2 3 Starting at (0, 4), plot another point by moving right 3 units and down 2 units. 3 Draw a line through the two points. Slope-Intercept Form 8.5 LESSON

5. 5 Real-Life Situations In a real-life problem involving a linear equation, the y -intercept is often an initial value, and the slope is a rate of change. Slope-Intercept Form 8.5 LESSON

6. 6 Using Slope and y -intercept in Real Life Earth Science The temperature at Earth’s surface averages about 20˚C. In the crust below the surface, the temperature rises by about 25˚C per kilometer of depth. SOLUTION Let x be the depth (in kilometers) below Earth’s surface, and let y be the temperature (in degrees Celsius) at that depth. Write a verbal model. Then use the verbal model to write an equation. EXAMPLE 3 Write an equation that approximates the temperature below Earth’s surface as a function of depth. y = 20 + 25x Slope-Intercept Form 8.5 LESSON Temperature below surface = Temperature at surface + Rate of change In temperature Depth below surface

7. 7 Parallel and Perpendicular Lines There is an important relationship between the slopes of two nonvertical lines that are parallel or perpendicular. Slopes of Parallel and Perpendicular Lines Two nonvertical parallel lines have the same slope. For example, the parallel lines a and b below both have a slope of 2. Two nonvertical perpendicular lines, such as lines a and c below, have slopes that are negative reciprocals of each other. a || ba c Slope-Intercept Form 8.5 LESSON

8. 8 Finding Slopes of Parallel and Perpendicular Lines SOLUTION EXAMPLE 4 Find the slopes of the lines that are parallel and perpendicular to the line with equation 4x + 3y = –18. Subtract 4x from each side. 3y = –4x – 18 First write the given equation in slope-intercept form. 4x + 3y = –18 Write original equation. y = – x – 6 4343 Multiply each side by. 1313 The slope of the given line is –. Because parallel lines have the same slope, the slope of a parallel line is also –. The slope of a perpendicular line is the negative reciprocal of –, or. 4343 4343 4343 3434 Slope-Intercept Form 8.5 LESSON