✓ Got It?

4. 1. What is the equation of the line in Problem 4 if you use ( − 2 , − 1 ) open negative 2 comma negative 1 close  instead of (3, 5) in the last step?
   2. Rewrite the equations in Problem 4 and part (a) in slope-intercept form and compare them. What can you conclude?

Problem 5 Writing Equations of Horizontal and Vertical Lines

What are the equations for the horizontal and vertical lines through (2, 4)?

Every point on the horizontal line through (2, 4) has a y-coordinate of 4. The equation of the line is y = 4 . y equals 4 .  It crosses the y-axis at (0, 4).

Every point on the vertical line through (2, 4) has an x-coordinate of 2. The equation of the line is x = 2 . x equals 2 .  It crosses the x-axis at (2, 0).

✓ Got It?

5. 1. What are the equations for the horizontal and vertical lines through ( 4 , − 3 ) ? open 4 comma negative 3 close question mark
   2. Reasoning Can you write the equation of a vertical line in slope-intercept form? Explain.

✓ Lesson Check

Do you know HOW?

For Exercises 1 and 2, find the slope of the line passing through the given points.

1. (4, 5) and (6, 15)
2. 
3. What is an equation of a line with slope 8 and y-intercept 10?
4. What is an equation of a line passing through (3, 3) and (4, 7)?

Do you UNDERSTAND?

5. Vocabulary Explain why you think slope-intercept form makes sense as a name for y = mx + b . y equals mx . plus b .  Explain why you think point-slope form make sense as a name for y − y 1 = m ( x − x 1 ) . y minus , y sub 1 . equals m open . x minus , x sub 1 , close .
6. Compare and Contrast Graph y = 2 x + 5 y equals . 2 x plus 5  and y = − 1 3 x + 5 . y equals . negative , 1 third . x plus 5 .  Describe how these lines are alike and how they are different.

7. Error Analysis A classmate found the slope of the line passing through ( 8 , − 2 ) open 8 comma negative 2 close  and (8, 10), as shown below. Describe your classmate's error. Then find the correct slope of the line passing through the given points.

   m = 8 − 8 10 − ( − 2 ) m = 0 12 m = 0 table with 3 rows and 2 columns , row1 column 1 , m equals , column 2 fraction 8 minus 8 , over 10 minus open negative 2 close end fraction , row2 column 1 , m equals , column 2 0 twelfths , row3 column 1 , m equals , column 2 0 , end table