Perimeter and missing lengths for the 11 plus

Perimeter is the distance round the outside. Walk round it and add every side. A rectangle has two lengths and two widths. For L-shapes, find the missing sides first by using what you know: the totals must match.

Slide the notch's sides across. The two sides of the notch together are the same distance as the two sides they replaced. The notch just changes the route, not the length. The perimeter is still 2 times 12 plus 8, 40.

The trap is taking the notch off, 4 plus 3, to get 33. That treats the cut like lost area. Perimeter is a walk, so trace the actual edge, and the walk is just as long.

Part of the Geometry topics for the 11+.

• 6.1Properties of 2D shapesFold it in your head before you count
• 6.3AreaUse the height that stands up straight
• 6.43D shapesCount the edges you cannot see
• 6.5Volume and capacityVolume means multiply, never add
• 6.6Types of angle and measuringGuess first, then read the scale
• 6.7Angle factsKnow the total, find what is left
• 6.8Position and coordinatesAlong the corridor, then up the stairs

Every 11 plus maths topic