SOLUTION: how can I tell if two lines are perpendicular?

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Question 256201: how can I tell if two lines are perpendicular?
Found 2 solutions by nerdybill, drk:
Answer by nerdybill(7384) About Me  (Show Source):
You can put this solution on YOUR website!
1. Find the slope of each line
2. Multiply the two slopes together
3. If they equal -1, they are perpendicular
.
That is, two lines are perpendicular if their slopes are "negative reciprocal".

Answer by drk(1908) About Me  (Show Source):
You can put this solution on YOUR website!
find the slopes of the two lines. There are 3 ways to tell if they are perpendicular:
(1) graph then and if they cross at right angles, they are perpendicular.
(2) multiply the two slopes. If the product is -1, then they are perpendicular
(3) If the slope of one is the negative flip of the slope of the second, then they are perpendicular.