McGraw Hill Integrated II, 2012
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McGraw Hill Integrated II, 2012 View details
1. Bisectors of Triangles
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Exercise 42 Page 413

The perpendicular bisector of a segment is the perpendicular through its midpoint.

y=7/6x+8/3

Practice makes perfect

We have to find the equation of the perpendicular bisector for the segment whose endpoints are C(- 4,5) and D(2,- 2). We will do this in three steps.

  1. Find the midpoint of CD.
  2. Find the slope of the perpendicular bisector.
  3. Use the point-slope form to write the equation of the line and rewrite to obtain it in the slope-intercept form.

Let's go for it!

Midpoint of CD

A segment bisector contains the midpoint of the segment. We will use the Midpoint Formula to find the midpoint M of CD. M( x_1+x_2/2,y_1+y_2/2 )To find the coordinates of the midpoint M, we will substitute (- 4,5) and (2,- 2) for (x_1,y_1) and (x_2,y_2) in the above formula.
M( x_1+x_2/2,y_1+y_2/2 )
M( - 4+ 2/2,5+( - 2)/2 )
M( - 4+2/2, 5-2/2 )
M( - 2/2, 3/2 )
M(- 2/2, 3/2 )
M(- 1, 3/2 )
Therefore, M(- 1, 32) is the midpoint of CD.

Slope of the Perpendicular Bisector

A perpendicular bisector is perpendicular to the segment through the midpoint. In order to find the slope of the bisector, we will first find the slope of CD. To do so, we will use the Slope Formula. m = y_2-y_1/x_2-x_1 Let's substitute (- 4,5) and (2,- 2) for (x_1,y_1) and (x_2,y_2) in the above formula.
m = y_2-y_1/x_2-x_1
m=2-( - 4)/- 2- 5
m=2+4/- 2-5
m=6/- 7
m=- 6/7
The slope of CD is - 67. Let m_p be the slope of the perpendicular bisector. The product of the slopes of two perpendicular lines is - 1.
- 6/7 * m_p = - 1
6/7 * m_p = 1
7 * 6/7 * m_p = 7
6 * m_p = 7
m_p= 7/6
The slope of the perpendicular bisector of CD is 76.

Equation of the Perpendicular Bisector

Since we know a point and the slope of the bisector, we will use the point-slope form of a line to write its equation. y-y_1=m(x-x_1) Let's substitute 76 for m and the midpoint ( - 1, 32) for (x_1,y_1) in the formula. Later, we will rewrite the equation of the line to obtain it in the slope-intercept form.
y-y_1=m(x-x_1)
y- 3/2= 7/6(x-( - 1))
y-3/2=7/6(x+1)
y-3/2=7/6x+7/6
y=7/6x+7/6+3/2
y=7/6x+7/6+3 * 3/2 * 3
y=7/6x+7/6+9/6
y=7/6x+16/6
y=7/6x+8/3
In slope-intercept form, the equation of the perpendicular bisector of the segment whose endpoints are C(- 4,5) and D(2,- 2) is y= 76x+ 83.