4 Write Equations of Horizontal and Vertical Lines In Section 4.2, we learned that the slope of a horizontal line is zero and that it has equation y b, where b is a constant. The slope of a vertical line is undefi ned, and its equation is x a, where a is a constant. Formula Equations of Horizontal and Vertical Lines Equation of a Horizontal Line: The equation of a horizontal line containing the point (a, b) is y b. Equation of a Vertical Line: The equation of a vertical line containing the point (a, b) is x a. Notice that the equation of a horizontal line has only a y-variable term, and a vertical line has only an x-variable term. EXAMPLE 4 Write an equation of the horizontal line containing the point (5, 4). Solution The equation of a horizontal line has the form y b, where b is the y-coordinate of the point. The equation of the line is y 4. YOU TRY 4 Write an equation of the horizontal line containing the point (1, 7). Summary Writing Equations of Lines If you are given 1) the slope and y-intercept of the line, use y mx b and substitute those values into the equation. 2) the slope of the line and a point on the line, use the point-slope formula: y y1 m(x x1) Substitute the slope for m and the point you are given for (x1, y1). Write your answer in slope-intercept or standard form. 3) two points on the line, fi nd the slope of the line and then use the slope and either one of the points in the point-slope formula. Write your answer in slope-intercept or standard form. The equation of a horizontal line containing the point (a, b) is y b. The equation of a vertical line containing the point (a, b) is x a. 5 Use Slope to Determine if Two Lines Are Parallel or Perpendicular Recall that two lines in a plane are parallel if they do not intersect. If we are given the equations of two lines, how can we determine if they are parallel? Here are the equations of two lines, L1 and L2. L1: y 2 3 x 1 L2: y 2 3 x 5 Outline this summary, in your own words, in your notes. www.mhhe.com/messersmith SECTION 4.3 Writing an Equation of a Line 175
messersmith_power_intermediate_algebra_1e_ch4_7_10
To see the actual publication please follow the link above