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# Mathematics Matters Lesson Accounts 45 - Angles of a Polygon

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Created on 29 May 2008 by ncetm_administrator
Updated on 17 June 2008 by ncetm_administrator
 Mathematics Matters Lesson Accounts A collection of memorable mathematics lessons that conference and colloquia delegates had observed or taught which they felt were successful.  Each account refers to one or more of the values and principles in the report.

# Lesson Account 45 - Angles of a Polygon

 Written by Richard Deakin Organisation Failsworth School Age/Ability Range Year 9 level 5/6 students

Task to build on knowledge of external angles of a polygon. This came after lesson on internal angles. Teacher explained what external angle was. The main task was to create an irregular polygon with an even number of sides. with three exterior angles of 90°, 60°, 90°, and the remaining exterior angles being equal. Pupils asked (i) how many sides could the polygon have, (ii) to work out the missing angles and make sketches of what the different polygons could look like.

Pupils discussed how many sides polygon had. Small groups sketched shape on plan paper. An absolute mess was created with pupils’ attempts.

Successful solution was obtained by pupils and shared with other groups.

What were the critical moments?
When pupils realised that internal + external angle = 180°, so if external angle was 120°, the internal angle must be 60°. Once pupils deduced the 3 internal angles the solution became easier to obtain.

What mathematics was learnt? (on plan and off plan) and what is the evidence of learning?
Strengthened knowledge of polygons, internal and external angles

How was that mathematics learnt?
Through deductive reasoning. The initial task helped pupils to understand external and internal angle = 180°, and external angles in polygon added to 360°, .

Other memorable outcomes
Solutions displayed on classroom walls.

Resources
RM Kaleidos maths alive http://www.learningplatform.rm.com/,
Y9 core SSM1 lesson 7 (tasks based from task in this product).

## Values & Principles

 Strategies for investigation and problem solving Makes appropriate use of whole class interactive teaching, individual work and cooperative small group work Uses rich, collaborative tasks

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