Constructing triangles
I can use intersecting circles to construct triangles from given lengths.
- Year 8
Constructing triangles
I can use intersecting circles to construct triangles from given lengths.
Lesson details
Key learning points
- A construction is an accurate drawing made using geometrical properties.
- A scalene triangle can be constructed using intersecting circles with different radii.
- An equilateral triangle can be constructed using intersecting circles with equal radii.
- An isosceles triangle can be constructed using intersecting circles.
Keywords
Scalene triangle - Scalene triangles have three different-sized angles and three edges of different lengths.
Isosceles triangle - Isosceles triangles have two equal-sized angles. Two of the edges are equal in length.
Equilateral triangle - Equilateral triangles have three equal-sized angles and three edges of equal length. They are regular triangles.
Radius - The radius is any line segment that joins the centre of a circle to any point on its circumference.
Common misconception
Pupils may avoid using the pair of compasses and opt for a ruler instead.
Emphasise the efficiency and accuracy of using compasses instead of a ruler in the construction of their triangles.
Licence
Prior knowledge starter quiz
6 Questions
Q1.Match the angle size to the type of angle.
$$12^\circ$$ -Â acute angle
$$90^\circ$$ -Â right angle
$$138^\circ$$ -Â obtuse angle
$$305^\circ$$ -Â reflex angle
Q2.Which of these sketched triangles are definitely isosceles?




Q3.Lucas has used a centimetre ruler to draw a line of 8.2 cm. Which image shows his line?




Q4.Which of these shows the best set up of a pair of compasses?



Q5.An arc of a circle is part of the circle’s .
Q6.A triangle has a perimeter of 65 mm. Two of the edges are 23 mm and 26 mm respectively. How long is the third edge of the triangle?
Assessment exit quiz
6 Questions
Q1.Which description matches this sketch?

Q2.When constructing a triangle from three edge lengths, we use a pair of compasses to draw circles.
Q3.Which of the following shows a partial construction of an equilateral triangle?




Q4.Which of the following lengths could be the edge lengths of a triangle?
Q5.Which of the points C, D or E when connected to AB would create a triangle with three edges of different lengths?

Q6.Triangle ABE is an triangle.
