Warm Up Alice finds her flower bulbs multiply each year. She started with just 24 tulip plants. After one year she had 72 plants. Two years later she had 120. Find a linear function to model the growth of Alice’s bulbs.

Warm Up Find the slope of the line containing each pair of points. 1. (0, 2) and (3, 4) 2. (–2, 8) and (4, 2) 3. (3, 3) and (12, –15) Write the following equations in slope-intercept form. 4. y – 5 = 3(x + 2) 5. 3x + 4y + 20 = 0 –1 –2 y = 3x + 11

Objectives Graph a line and write a linear equation using point-slope form. Write a linear equation given two points.

Additional Example 1A: Writing Linear Equations in Point-Slope Form Write an equation in point slope form for the line with the given slope that contains the given point. y – y1 = m (x – x1) Write the point-slope form.

Additional Example 1B: Writing Linear Equations in Point-Slope Form Write an equation in point slope form for the line with the given slope that contains the given point. slope = –4; (0, 3) y – y1 = m(x – x1) Write the point-slope form. Substitute –4 for m, 0 for x1 and 3 for y1. y – 3 = –4(x – 0) y – 3 = –4(x – 0)

Additional Example 1C: Writing Linear Equations in Point-Slope Form Write an equation in point slope form for the line with the given slope that contains the given point. slope = 1; (–1, –4) y – y1 = m(x – x1) Write the point-slope form. Substitute 1 for m, –1 for x1, and –4 for y1. y – (–4) = 1(x – (–1)) Rewrite subtraction of negative numbers as addition. y + 4 = 1(x + 1)

Check It Out! Example 1a Write an equation in point slope form for the line with the given slope that contains the given point. y – y1 = m(x – x1) Write the point-slope form. Substitute 2 for m, for x1 and 1 for y1.

Check It Out! Example 3a Continued Write an equation in slope-intercept form for the line with slope that contains (–3, 1). Step 2 Write the equation in slope-intercept form by solving for y. Rewrite subtraction of negative numbers as addition. Distribute on the right side. +1 +1 Add 1 to both sides.

Check It Out! Example 3b Write an equation in slope-intercept form for the line through the two points. (1, –2) and (3, 10) Step 1 Find the slope. Step 2 Substitute the slope and one of the points into the point-slope form. y – y1 = m(x – x1) y – (–2) = 6(x – 1) Choose (1, –2). y + 2 = 6(x – 1)

Check It Out! Example 3b Continued Write an equation in slope-intercept form for the line through the two points. (1, –2) and (3, 10) Step 3 Write the equation in slope-intercept form. y + 2 = 6(x – 1) Distribute 6 on the right side. y + 2 = 6x – 6 – 2 – 2 Subtract 2 from both sides. y = 6x – 8

Check It Out! Example 4 Find the x- and y-intercepts of the line that passes through the points: (2, 15) and (–4, –3) Step 1 Find the slope. Step 2 Write the equation in slope-intercept form. y – y1 = m(x – x1) Choose (2, 15). y − 15 = 3(x − 2) Distribute 3 on the right side. y − 15 = 3x − 6 Add 15 to both sides. y = 3x + 9

Check It Out! Example 4 Continued Step 3 Find the intercepts. x-intercept: y-intercept: Use the slope-intercept form to indentify the y-intercept. Replace y with 0 and solve for x. y = 3x + 9 y = 3x + 9 0 = 3x + 9 b = 9 –9 = 3x –3 = x The x-intercept is –3, and the y-intercept is 9.

Example 5: Problem-Solving Application The cost to stain a deck is a linear function of the deck’s area. The cost to stain 100, 250, 400 square feet are shown in the table. Write an equation in slope-intercept form that represents the function. Then find the cost to stain a deck whose area is 75 square feet.

Example 5 Continued Understand the Problem 1 • The answer will have two parts—an equation in slope-intercept form and the cost to stain an area of 75 square feet. • The ordered pairs given in the table—(100, 150), (250, 337.50), (400, 525)—satisfy the equation.

Example 5 Continued 2 Make a Plan You can use two of the ordered pairs to find the slope. Then use point-slope form to write the equation. Finally, write the equation in slope-intercept form.

Example 5 Continued Solve 3 Step 1 Choose any two ordered pairs from the table to find the slope. Use (100, 150) and (400, 525). Step 2 Substitute the slope and any ordered pair from the table into the point-slope form. y – y1 = m(x – x1) y – 150 = 1.25(x – 100) Use (100, 150).

Example 5 Continued Solve 3 Step 3 Write the equation in slope-intercept form by solving for y. y – 150 = 1.25(x – 100) y – 150 = 1.25x – 125 Distribute 1.25. y = 1.25x + 25 Add 150 to both sides. Step 4 Find the cost to stain an area of 75 sq. ft. y = 1.25x + 25 y = 1.25(75) + 25 = 118.75 The cost of staining 75 sq. ft. is $118.75.

  Example 5 Continued Look Back 4 If the equation is correct, the ordered pairs that you did not use in Step 2 will be solutions. Substitute (400, 525) and (250, 337.50) into the equation. y = 1.25x + 25 337.50 1.25(250) + 25 337.50 312.50 + 25 337.50 337.50  y = 1.25x + 25 525 1.25(400) + 25 525 500 + 25 525 525 

Lesson Quiz: Part I Write an equation in slope-intercept form for the line with the given slope that contains the given point. 1. Slope = –1; (0, 9) y − 9 = –(x − 0) y + 6 = (x – 3) 2. Slope = ; (3, –6) Write an equation that describes each line the slope-intercept form. 3. Slope = –2, (2, 1) is on the line y = –2x + 5 y = x + 4 4. (0, 4) and (–7, 2) are on the line

Lesson Quiz: Part II 5. The cost to take a taxi from the airport is a linear function of the distance driven. The cost for 5, 10, and 20 miles are shown in the table. Write an equation in slope-intercept form that represents the function. y = 1.6x + 6