Perimeter and Area

Students learn how to find the perimeter and area of various 2D shapes including triangles, quadrilaterals, compound shapes and circles.  Throughout the topic links are made to algebraic reasoning and estimation.  This topic takes place in Term 4 of Year 9 and links to arc length and area of sectors later on.

Perimeter and Area Lessons

Prerequisite Knowledge

  • choose and use appropriate standard units to estimate and measure length/height in any direction (m/cm); mass (kg/g); temperature (°C); capacity (litres/ml) to the nearest appropriate
  • unit, using rulers, scales, thermometers and measuring vessels
  • compare and order lengths, mass, volume/capacity and record the results using >, < and =
  • measure, compare, add and subtract: lengths (m/cm/mm); mass (kg/g); volume/capacity (l/ml)

Success Criteria

know and apply formulae to calculate:

  • rectangles
  • rectilinear composite shapes
  • area of triangles
  • parallelograms
  • trapezia

calculate the perimeters of 2D shapes, including composite shapes;

Key Concepts

  • Perimeter is a measure of length around the outside of a shape whereas area is the space on a surface.
  • Students should be given the opportunity to derive formulae to calculate the area and perimeter of shapes.
  • Area is measured in square units.
  • Demonstrate a triangle as being half a rectangle so students know to use the perpendicular height in their calculation.
  • Demonstrate a parallelogram as having an equal area to a rectangle.
  • To calculate the area of composite rectilinear shapes have students break them up in different ways.

Common Misconceptions

  • Students often confuse area and perimeter.
  • Students can forget to include correct units when stating an area or perimeter.
  • When calculating the area of a triangle or parallelogram students tend to use the slanted height rather than the correct perpendicular height.

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