Tuesday, 24 March 2009


Take a square. There are four equal sides. Let's call the length of the side s, so the perimeter of the square has a length of 4s. If you take a rectangle there is a length and width. Call the length l and the width w. The perimeter of the rectangle is 2l+2w or 2(l+w). All dimensions including length and width have to have units. this may be a metric unit like centimetres and metres or an imperial unit like inches and feet.

It gets more complicated with circles. Let's call the diameter d, and the radius is r. If you draw a circle you will see that the radius is half the diameter. In this case the perimeter of the circle is found by the formula 2πr.

π (pi, pronounced like 'pie') is a fixed number, which can be found by measuring or by calculation. It's a little over 3. To three decimal places it's 3.142. To 8 it's 3.14159265. It can never be expressed exactly either as a fraction or as a decimal, so in calculations you just have to use enough decimal places to get as close to the real answer as you need to get.

The area of the square of side s is sxs. The area of a rectangle of length l and width w is lw. The formula for the area of a circle is πd where d is the diameter. Whichever units you are using the units for an area is a lenth x lenth. If you are dealing with centimetres, the units of the area are centimetres squared.

That sums it up

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