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Write an equation given the slope and a point EXAMPLE 2 Write an equation of the line that passes through (5, 4) and has a slope of – 3. Because you know.

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Presentation on theme: "Write an equation given the slope and a point EXAMPLE 2 Write an equation of the line that passes through (5, 4) and has a slope of – 3. Because you know."— Presentation transcript:

1 Write an equation given the slope and a point EXAMPLE 2 Write an equation of the line that passes through (5, 4) and has a slope of – 3. Because you know the slope and a point on the line, use point-slope form to write an equation of the line. Let (x 1, y 1 ) = (5, 4) and m = – 3. y – y 1 = m(x – x 1 ) Use point-slope form. y – 4 = – 3(x – 5) Substitute for m, x 1, and y 1. y – 4 = – 3x + 15 Distributive property SOLUTION y = – 3x + 19 Write in slope-intercept form.

2 EXAMPLE 3 Write an equation of the line that passes through (–2,3) and is (a) parallel to, and (b) perpendicular to, the line y = –4x + 1. SOLUTION a. The given line has a slope of m 1 = –4. So, a line parallel to it has a slope of m 2 = m 1 = –4. You know the slope and a point on the line, so use the point- slope form with (x 1, y 1 ) = (– 2, 3) to write an equation of the line. Write equations of parallel or perpendicular lines

3 EXAMPLE 3 y – 3 = – 4(x – (– 2)) y – y 1 = m 2 (x – x1) Use point-slope form. Substitute for m 2, x 1, and y 1. y – 3 = – 4(x + 2) Simplify. y – 3 = – 4x – 8 Distributive property y = – 4x – 5 Write in slope-intercept form. Write equations of parallel or perpendicular lines

4 EXAMPLE 3 b. A line perpendicular to a line with slope m 1 = – 4 has a slope of m 2 = – =. Use point-slope form with (x 1, y 1 ) = (– 2, 3) m1m1 y – y 1 = m 2 (x – x1) Use point-slope form. y – 3 = (x – (– 2)) 1 4 Substitute for m 2, x 1, and y 1. y – 3 = (x +2) 1 4 Simplify. y – 3 = x Distributive property Write in slope-intercept form. y = x Write equations of parallel or perpendicular lines

5 GUIDED PRACTICE for Examples 2 and 3 GUIDED PRACTICE 4. Write an equation of the line that passes through (– 1, 6) and has a slope of 4. SOLUTION Because you know the slope and a point on the line, use the point-slope form to write an equation of the line. Let (x 1, y 1 ) = (–1, 6) and m = 4 y – 6 = 4(x – (– 1)) y – y 1 = m(x – x1) Use point-slope form. Substitute for m, x 1, and y 1. y – 6 = 4x + 4 Distributive property y = 4x + 10 Write in slope-intercept form.

6 GUIDED PRACTICE for Examples 2 and 3 GUIDED PRACTICE 5. Write an equation of the line that passes through (4, –2) and is (a) parallel to, and (b) perpendicular to, the line y = 3x – 1. SOLUTION The given line has a slope of m 1 = 3. So, a line parallel to it has a slope of m 2 = m 1 = 3. You know the slope and a point on the line, so use the point - slope form with (x 1, y 1 ) = (4, – 2) to write an equation of the line. y – (– 2) = 3(x – 4) y – y 1 = m 2 (x – x 1 ) Use point-slope form. Substitute for m 2, x 1, and y 1. y + 2 = (x – 4) Simplify. y + 2 = 3x – 12 Distributive property y = 3x – 14 Write in slope-intercept form.

7 GUIDED PRACTICE for Examples 2 and 3 GUIDED PRACTICE y – y 1 = m 2 (x – x 1 ) Use point-slope form. Substitute for m 2, x 1, and y 1. Simplify. Distributive property Write in slope-intercept form. Use point - slope form with (x 1, y 1 ) = (4, – 2) y – (– 2) = – (x – 4) 1 3 y + 2 = – (x – 4) 1 3 y = – x – b. A line perpendicular to a line with slope m 1 = 3 has a slope of m 2 = – = – 1 m1m1 4 3 y + 2 = – x – 1 3


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