I°) Definition
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Two lines are perpendicular if they intersect at a right angle. |
| Let a point A and line D. There is a right and one from A and
perpendicular to D.
Notation : A _|_ B |
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If two lines are parallel, all perpendicular to one is perpendicular to another.
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| Two lines perpendicular to a third are parallel to each. |  |
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