Starter quiz
- Angles that meet at a single point on a straight line sum to ______°.
- '180' ✓
- Angles are __________ if they sum to 180°.
- conjugate
- equal
- similar
- supplementary ✓
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- Two angles meet to form a straight line. One angle is 60°. The other angle is ______°.
- '120' ✓
- Squares and equilateral triangles are examples of __________ polygons.
- congruent
- irregular
- regular ✓
- similar
-
- A regular octagon has an edge with length 5 cm. Therefore, its perimeter is ______ cm.
- '40' ✓
- A regular decagon has a perimeter of 40 cm. Therefore, each of its edges are of length ______ cm.
- '4' ✓
Exit quiz
- Two of the interior angles of a triangle are 80° and 45°. The size of the third interior angle is ______°.
- '55' ✓
- The interior angles of a pentagon sum to ______°.
- '540' ✓
- Each interior angle of a regular hexagon is ______°.
- '120' ✓
- A regular polygon has interior angles of size 150°. The polygon has ______ sides.
- '12' ✓
- Which of these interior angles are possible in a regular polygon?
- 72°
- 135° ✓
- 140° ✓
- 144° ✓
- 180°
-
- One of the interior angles in an isosceles triangle is 70°. Which are possible sizes for one of the other interior angles?
- 25°
- 40° ✓
- 55° ✓
- 70° ✓
- 85°
-
Worksheet
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Presentation
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Video
Lesson Details
Key learning points
- Any polygon can be split into triangles.
- Using these triangles you can demonstrate the rule for the interior angle sum of a polygon.
- A missing interior angle in any polygon can be found.
- The number of sides in a regular polygon can be determined from the size of the interior angle.
- The sum of the interior angles is the same, regardless of whether the polygon is regular.
Common misconception
The size of an interior angle in a regular shape cannot be found unless the size of the other angles are given.
A regular shape has the property that all of its interior angles are equal in size. This means that the size of each interior angle is very easy to find if, for example, the sum of the interior angles is known.
Keywords
Polygon - A polygon is a flat (2D), closed figure made up of straight line segments.
Regular - A regular polygon has sides that are all equal and interior (inside) angles that are all equal.
Interior angle - An interior angle is an angle formed inside a polygon by two of its edges.
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