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January 29, 2015

January 29, 2015

Posted by **Anonymous** on Wednesday, March 12, 2008 at 11:09am.

20) For each of the following, find a normal vector, a direction vector, and a point of each line.

Note, normal vector means perpendicular to the direction vector.

a) 3x - 6y = 14

TEXTBOOK ANSWER:

normal vector = (1 , -2)

direction vector = (2 , 1)

point on the line = [(14/3) , 0]

-----------

21) Find vector, parametric, and symmetric equations of the following lines.

a) -4x + 6y + 9 = 0

TEXTBOOK ANSWER:

Vector Equation -> r = [0 , (-3/2)] + t(3 , 2)

Parametric Equation -> x = 3t and y = 2t - (3/2)

Symmetric Equation -> (x/3) = [y + (3/2)] / 2

- Discrete Math: Equations of Line in a Plane -
**Reiny**, Wednesday, March 12, 2008 at 12:32pmI will do the second one, since it looks a bit more difficult than the first

You are clearly working in only 2-D here.

I assume you know that for any straight line

Ax + By + C = 0

[A,B] is a normal to the line which makes

[B,-A] a direction vector

and the slope of the line is -A/B

the vector equation of a line in 2-D is

r = [any point on the line] + t[direction vector of the line], where t is a parameter.

so they started by letting x=0, then y = -9/6 = -3/2

so the point they used is (0,-3/2)

and the direction of the line is [6,4] or [3,2]

so the vector equation is

r = [0,-3/2] + t[3,2] as shown in your text.

if we "expand" that we get

r = [0+3t,-3/2 + 2t]

from that you can see that the

x=3t and the

y=-3/2 + 2t as shown in your book answer

now if we solve these last two equations for t we get

t = x/3 and t = (y+3/2)/2

equating t=t gives us their third answer of

x/3 = (y+3/2)/2

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