New
New
Year 8

Constructing triangles

I can use intersecting circles to construct triangles from given lengths.

New
New
Year 8

Constructing triangles

I can use intersecting circles to construct triangles from given lengths.

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Lesson details

Key learning points

  1. A construction is an accurate drawing made using geometrical properties.
  2. A scalene triangle can be constructed using intersecting circles with different radii.
  3. An equilateral triangle can be constructed using intersecting circles with equal radii.
  4. 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.

Random number generators, dice or playing cards can be used to select three edge lengths for pupils to construct. Pupils can consider if a triangle can be constructed before attempting to.
Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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6 Questions

Q1.
Match the angle size to the type of angle.
Correct Answer:$$12^\circ$$,acute angle

acute angle

Correct Answer:$$90^\circ$$,right angle

right angle

Correct Answer:$$138^\circ$$,obtuse angle

obtuse angle

Correct Answer:$$305^\circ$$,reflex angle

reflex angle

Q2.
Which of these sketched triangles are definitely isosceles?
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
Q3.
Lucas has used a centimetre ruler to draw a line of 8.2 cm. Which image shows his line?
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
An image in a quiz
Q4.
Which of these shows the best set up of a pair of compasses?
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
Q5.
An arc of a circle is part of the circle’s .
Correct Answer: circumference
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?
15 mm
Correct answer: 16 mm
17 mm
18 mm
19 mm

6 Questions

Q1.
Which description matches this sketch?
An image in a quiz
Triangle PQR, where PQ = 7 cm, PR = 12 cm and QR = 14 cm
Triangle PQR, where PQ = 7 cm, PR = 14 cm and QR = 12 cm
Correct answer: Triangle PQR, where PQ = 12 cm, PR = 14 cm and QR = 7 cm
Triangle PQR, where PQ = 14 cm, PR = 7 cm and QR = 12 cm
Triangle PQR, where PQ = 14 cm, PR = 12 cm and QR = 7 cm
Q2.
When constructing a triangle from three edge lengths, we use a pair of compasses to draw circles.
Correct Answer: intersecting
Q3.
Which of the following shows a partial construction of an equilateral triangle?
An image in a quiz
An image in a quiz
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
Q4.
Which of the following lengths could be the edge lengths of a triangle?
Correct answer: 3 cm, 4 cm and 5 cm
3 cm, 3 cm and 6 cm
Correct answer: 8 cm, 8 cm and 8 cm
7 cm, 9 cm and 18 cm
Correct answer: 9 cm, 9 cm and 14 cm
Q5.
Which of the points C, D or E when connected to AB would create a triangle with three edges of different lengths?
An image in a quiz
Correct answer: C
D
E
Q6.
Triangle ABE is an triangle.
An image in a quiz
Correct Answer: isosceles