Copy a corner, no protractor
Show on a big screenA tailor copies a corner onto cloth. No protractor, the tool that measures angles. The two lines of a corner are its arms.
An arc is part of a circle. To strike an arc is to draw it with a compass.
- The corner to copy: two arms.
- Strike an arc. It leaves two marks, one on each arm.
- Rule a base line with a ruler. Strike an arc, same radius.
- Carry the gap between the two marks. It fixes the second arm.
Same radius, same gap
Strike an arc from the corner. It crosses the arms at B and C. Measure the gap BC with the compass. The gap between its point and its pencil is the span.
Set the span back to the first radius. Strike an arc of the same radius from the new corner. Carry the gap BC onto it. Put the compass point where the new arc meets the base line. Keep the span BC. Mark that distance on the new arc. The new arm goes through that mark.
Each corner and its two marks make a triangle. The two triangles have three equal pairs of sides. So the two corners are equal.
Before you touch a compass, ask an adult. The point is sharp.
Change the radius of the new arc. Watch the two corners.
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Try the lab to finish this lesson.