How can you determine if two lines are perpendicular?

Question

How can you determine if two lines are perpendicular?

Answer 1

Two lines are perpendicular if the product of their slopes is -1.

Just multiply their slopes.  

Answer 2

Put a protractor next to them and see if it reads 90 degrees in the angle they make.    If you have no protractor, you could mark out an equal distance on each side of one line, call it the horizontal line, (on opposite sides of the other line)   and measure to a point on the vertical line.   If it is equal in distance, they should be perpendicular.  You can use a compass, also, and swing some arcs.

Answer 3

if their slope is the same and they are in the same plane than the lines are either equal or parallel.

sorry, read the question wrong.  the answer above me is right.

Answer 4

If you have two equations for lines (like for graphing in Algebra I):

First determine the slopes of the two lines.

Then negate one of the slopes (e.x.: 5/3   -->   -5/3).

Then invert it. (e.x.: -5/3   -->   3/-5).

If the resulting slope and that of the other line are equal, the lines are perpendicular.

Example:

Equation One: y = 3x + 4

Equation Two: y = -2x - 1

Slope 1: 3

Slope 2: -2 --> 2 --> 1/2

3 =/= 1/2

Lines are not perpendicular.

Answer 5

If they intersect each other they will form a 90 degree (right) angle.