Maths · Mensuration. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A compound shape is made from simple shapes joined together, or from a simple shape with a piece cut out. Split it into parts you know: rectangles, triangles, circles, sectors, cylinders, cones and hemispheres.
For area or volume, add the parts that are joined and subtract the parts that are removed. A hole, a ring or a shaded region is nearly always the big shape minus the small shape.
Perimeter and surface area need more care. Count only the edges or faces on the outside. Where two parts are joined, the joining edge or face is hidden and is not counted. A semicircle has half the area of its circle, and a hemisphere has half the volume of its sphere.
In a perimeter, students include the line where two parts join, or leave out an outside edge that has no length written on it. Go round the outside once with your pen and tick each edge as you count it.
Answer: 320 cm3
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