ZIMSEC O Level · Technical Graphics and Design · Form 1, Form 2
Basic constructions (ruler and compasses)
- Bisect a line: arcs of equal radius from both ends; join the intersections.
- Bisect an angle: arc from the vertex, then equal arcs from where it cuts the arms.
- Construct 90°, 60°, 45° and 30° angles; draw parallel and perpendicular lines.
- Divide a line into equal parts using a sloping line and parallel lines.
Triangles and polygons
- Construct triangles from given sides and angles.
- Regular polygons: a hexagon by stepping off the radius six times around a circle; other polygons using the angle at the centre (360° ÷ number of sides).
Circles and tangents
- Find the centre of a circle by bisecting two chords.
- A tangent touches a circle at one point and is perpendicular to the radius at that point.
- Draw tangents to one or two circles; blend lines and arcs smoothly (find the centres of blending arcs).
Inscribed and circumscribed circles
Inscribed circle of a triangle: centre where the angle bisectors meet. Circumscribed circle: centre where the perpendicular bisectors of the sides meet.
Exam tip: Leave all construction arcs visible – they show your method and earn marks.
Check yourself
- How do you find the centre of a given circle?
- Where is the centre of a triangle's inscribed circle?
- What is the angle at the centre for a regular pentagon?
Show answers
- Draw two chords and bisect each; the bisectors meet at the centre.
- Where the bisectors of the angles meet.
- 72°
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