Parallels from square corners
Show on a big screenSlide a set square along a ruler. Draw a line each time. Are those lines parallel? Try the same idea by folding paper. A fold leaves a line called a crease.
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- Line p, and a point B above it.
- Fold 1: a crease u through B, square to p.
- Fold 2: a crease w through B, square to u.
- Arrow marks show that p and w are parallel.
The rule
Yes: lines drawn with a set square along one ruler are parallel.
On a flat page, two lines perpendicular to one line are parallel. Mark parallel lines with arrows. Mark a right angle with a small square.
Why? Both lines make 90° with that one line. Those are corresponding angles, and they are equal. So the lines are parallel.
A set square works the same way. Hold one edge against a ruler and slide it along. Lines drawn along the other edge are perpendicular to the ruler. So the lines are parallel to each other.
Watch out!
"Two lines that never meet are parallel." Not always. Two pencils held at different heights may never meet. Parallel lines lie on one flat surface.
Try the lab to finish this lesson.