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Maths

Loci

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Drawing triangles

There are two methods of drawing triangles - construction or using a ruler and protractor.

Construction

Construct a triangle with side lengths of 6cm, 5cm and 4cm.

Solution

  1. Use a ruler to draw a 6cm line. Label one end A and the other B.
  2. Open the compass to a radius of 5cm.
  3. Place the compass needle at point A and draw an arc above the line.
  4. Open the compass to a radius of 4cm.
  5. Move the compass needle to point B and draw an arc above it.
  6. Join each end of the line to the point where the arcs cross.

Remember, do not erase any construction arcs when using this method.

Ruler and protractor example

Some triangles are better to draw using a ruler and protractor.

Draw a triangle with one 6cm side, and two angles of 55° 70°.

Solution

  1. Use a ruler to draw a 6cm line. Label one end A and the other B.
  2. Place the centre of the protractor at point A, taking care to line it up at zero degrees.
  3. Measure an angle of 55° and mark it with a dot.
  4. Join point A to the dot with a ruler.
  5. Move the protractor to point B.
  6. Measure an angle of 70° and mark the angle with a dot.
  7. Join point B to the dot with a ruler.
  8. If the lines don't meet use a ruler to make them longer until they do.

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