Question

Geometry (21) Study Guide

Answer 1

Answer: 11x-3y = 32

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Explanation:

The given points are
A = (-4,-2)
B = (4,4)
C = (18,-8)

First we'll use points A and C to find the slope of line AC
m = (y2-y1)/(x2-x1)
m = (-8-(-2))/(18-(-4))
m = (-8+2)/(18+4)
m = -6/22
m = -3/11
The slope of AC is -3/11

Take the negative reciprocal of this slope
Flip the fraction: -3/11 -----> -11/3
Flip the sign: -11/3 ----> +11/3 = 11/3

The slope of AC is -3/11 while the slope of any line perpendicular to AC is 11/3
Let m = 11/3 and (x,y) = (4,4) which are the coordinates of point B
Plug these values into slope intercept form and then solve for b
y = mx+b
4 = (11/3)*4+b
4 = 44/3+b
4 - 44/3 = 44/3+b-44/3
b = 4 - 44/3
b = 12/3 - 44/3
b = (12 - 44)/3
b = -32/3

Since m = 11/3 and b = -32/3, we go from this
y = mx+b
to this
y = (11/3)x-32/3

Now clear out the fractions and get the x and y variables to one side
y = (11/3)x-32/3
3y = 3*[ (11/3)x-32/3 ]
3y = 11x - 32
3y-11x = 11x-32-11x
-11x+3y = -32
-1*(-11x+3y) = -1*(-32)
11x-3y = 32

The equation of the through B that is perpendicular to AC is 11x-3y = 32 (this equation is in Ax+By = C form which is called standard form)
This is better known as the altitude through B