How do you write the equation of a line given (5.2), (-7,-4)?

1 Answer
Mar 1, 2017

(y - color(red)(2)) = color(blue)(1/2)(x - color(red)(5))

Or

(y + color(red)(4)) = color(blue)(1/2)(x + color(red)(7))

Or

y = color(red)(1/2)x - color(blue)(1/2)

Explanation:

First, we need to determine the slope of the line. The slope can be found by using the formula: m = (color(red)(y_2) - color(blue)(y_1))/(color(red)(x_2) - color(blue)(x_1))

Where m is the slope and (color(blue)(x_1, y_1)) and (color(red)(x_2, y_2)) are the two points on the line.

Substituting the values from the points in the problem gives:

m = (color(red)(-4) - color(blue)(2))/(color(red)(-7) - color(blue)(5)) = (-6)/-12 = 1/2

Now, we can use the point-slope formula to write an equation for the line going through the two points. The point-slope formula states: (y - color(red)(y_1)) = color(blue)(m)(x - color(red)(x_1))

Where color(blue)(m) is the slope and color(red)(((x_1, y_1))) is a point the line passes through.

Substituting the slope we calculated and the first point gives:

(y - color(red)(2)) = color(blue)(1/2)(x - color(red)(5))

We can also substitute the slope we calculated and the second point giving:

(y - color(red)(-4)) = color(blue)(1/2)(x - color(red)(-7))

(y + color(red)(4)) = color(blue)(1/2)(x + color(red)(7))

Or, we can solve for y to put the equation in slope-intercept form. The slope-intercept form of a linear equation is: y = color(red)(m)x + color(blue)(b)

Where color(red)(m) is the slope and color(blue)(b) is the y-intercept value.

y + color(red)(4) = (color(blue)(1/2) xx x) + (color(blue)(1/2) xx color(red)(7))

y + color(red)(4) = 1/2x + 7/2

y + color(red)(4) - 4 = 1/2x + 7/2 - 4

y + 0 = 1/2x + 7/2 - (2/2 xx 4)

y = 1/2x + 7/2 - 8/2

y = color(red)(1/2)x - color(blue)(1/2)