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Imagine a triangle made by connecting the dots on the
circumference or in the centre of the circle.
Without pointing, how can you describe your triangle?
Here are some examples.
How would you describe them to your partner without
pointing?
How many different triangles made by connecting the
dots on the circumference or in the centre of the circle
can you draw?
How do you know you have got them all?
Look at the types of triangle you have. How can you
classify them?
How did this person classify their triangles?
Scalene triangle:
Nothing special about this triangle 
Right angled triangle:
1 angle of 90°
Right angled isosceles triangle:
1 angle of 90°
2 other equal angles
Equilateral triangle:
All sides equal
All angles are equal – all 60°
Isosceles triangle:
2 equal sides
2 equal angles
Isosceles triangle: Two equal sides, two equal angles
Same legs
(sides)
Same feet
(angles)
How do you know that this is an isosceles triangle
without measuring it?
How can I find the angle at the centre in this isosceles
triangle?
One student drew these lines.
What calculation would they do to find the angle, x?
Another student drew these lines.
What calculation would they do to find the angle, x?
Here’s another way.
What calculation would they do to find the angle, x?
How can I find the other angles in this isosceles triangle?
30°
x°
x°
How can I find the angles in this isosceles triangle?
(Try adding some lines to help)
On your whiteboards:
Find the missing angles
Explain how you found the answer
x°
20°
60°
x°
x°
x°
In your books:
Find the missing angles
Challenge questions
On your whiteboards:
Draw an isosceles triangle with all acute angles
and another
On your whiteboards:
Draw an isosceles triangle with an obtuse angle
and another