M3.5.5 For the adult
Drawing with ruler and compasses
I know the three sides of a triangle. Can I draw it exactly with a ruler and compasses?
What this chapter teaches
A ruler and compasses can draw a triangle exactly. With three sides, two arcs cross at the third corner. With two sides and the angle between them, a protractor marks the angle and an arc marks the length. Equal arcs from P and Q cut an angle in half. Compasses have a sharp point, so stay near.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why do the two arcs cross at the third corner of the triangle?
- Why can you not draw a triangle with sides of 2 cm, 3 cm and 7 cm?
- How do you check that the line you drew cuts the angle exactly in half?
After the chapter, the learner can
- Draw a triangle from three sides using compasses
- Draw a triangle from two sides and the angle between them
- Cut an angle exactly in half using compasses
What they copy into their notebook
Three sides: draw the base, then two arcs that cross at the third corner. Two sides and an angle: mark the angle, then use an arc for the length. Arcs from P and Q cut an angle in half.