# Introducing Angles in a Triangle

Building on angles on a straight line and vertically opposite angles students are taught why angles in a triangle have a sum of 180°.  More able students are challenged to prove this using angles in parallel lines.The start of the lesson recaps related angle properties such as those on a straight line and about a point.  The main part introduces the sum of angles in a triangle as 180° and provides several examples for practice.  The plenary challenges students to combine multiple properties within a single problem.
##### Differentiated Learning Objectives
• All students should be able to calculate angles in a scalene triangle.
• Most students should be able to calculate angles in an isosceles triangle and combine other angle properties.
• Some students should be able to prove angles in a triangle have a sum of 180°.

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