1 Parallel Lines

Parallel Lines

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This is almost where Geometry starts. One line. A point on it. At that point on one side of the line you have 180 degrees. 

Then two lines cross each other. And you have four angles. The opposite angles are equal

180 degrees

When two lines cross, the opposite angles are equal. Keep applying this rule in all of geometry and you do not really need to learn any of the parallel line theorems apart from the first i.e. Corresponding Angles being equal.
Corresponding Angles
It simply means two angles which are equal and which are made by the same lines. You must realize that two parallel lines are aligned the same way with its counterpart. So if another line crosses it, the angles made by it are going to be same. 

More Corresponding Angles

The Internal Angles

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In parallel lines the internal angles add up to 180 but again you do not need to memorize this. What we suggest is that whenever there are parallel lines identify two things

1. Corresponding Angles and

2. The Z or N angles as shown here

This should suffice. 


Why does the Three angles of a Triangle add up to 180 degrees