# Impossible triangles

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## Lesson details

### Key learning points

- In this lesson, we will determine when it is impossible to construct a triangle given three lengths.

### Licence

This content is made available by Oak National Academy Limited and its partners and licensed under Oak’s terms & conditions (Collection 1), except where otherwise stated.

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### 5 Questions

Q1.

If you know the length of the sides of a triangle, you can _______ it using a pair of ______.

construct, protractor

draw, compass

make,ruler

Q2.

This circle is drawn on a centimetre square grid. State the radius and diameter.

Radius = 2cm, Diameter = 2cm

Radius = 4cm, Diameter = 2cm

Q3.

This circle is drawn on a centimetre square grid. State the radius and diameter.

R = 1cm; D = 1cm

R = 2cm; D = 1cm

Q4.

This circle is drawn on a centimetre square grid. State the radius and diameter.

Others

R = 4cm, D = 4cm

R= 8cm, D = 4cm

Q5.

Two students were asked to construct a triangle with side lengths 3 cm , 2 cm and 4 cm. Are they both correct?

No, they are not both correct. Only B is correct because she mentioned the shortest angle first then measured the rest.

No, they are not both correct. Only student A is correct because the base is 3cm.

### 5 Questions

Q1.

When constructing a triangle, should you erase the construction lines at the end?

Some of them

Yes

Q2.

Which triangle is impossible to construct?

6cm, 8cm, 3cm

7cm, 3cm, 9cm

All of them

Q3.

What equipment would you use to construct a 9cm, 7cm, 18cm triangle correctly?

Ruler and pencil

Ruler, pencil and protractor

Q4.

Which triangle is impossible to construct?

1cm, 1cm, 1cm

6cm, 2cm, 5cm

Q5.

How do I know from the construction that this is an impossible triangle?

The base is 8cm. 4cm should have been the base.

The dotted line for 4cm is diagonal