Forming Equations from Shapes (Cambridge (CIE) IGCSE Maths)

Forming Equations from Shapes

How do I form equations from shapes?

• You need to use all the information given on the diagram and any specific properties of that shape

• Common 2D shapes that you should know properties for are

  • Triangles: equilateral, isosceles, scalene, right-angled

  • Quadrilaterals: square, rectangle, kite, rhombus, parallelogram, trapezium

• You may be asked about perimeter, area or angles

• You may be asked about polygons

  • Regular vs irregular polygons

  • Interior vs exterior angles

    • The sum of interior angles is 180(n-2) for an n-sided polygon

• You may be asked about angles in parallel lines

  • Alternative, corresponding and co-interior

• You may be asked about 3D shapes involving surface area and volume

  • Prisms have constant cross sections 

    • Volume is cross-section area multiplied by length

Is there anything else that can help?

• Sketch a diagram if none is given

• Split up uncommon shapes into the sum or difference of common shapes

• Look out for important extra information

  • For example, a trapezium "with a line of symmetry"

• With irregular shapes, assume all angles and lengths are different (unless told otherwise)

• Put brackets around algebraic expressions when substituting them into geometric properties