To calculate the perimeter, by definition, you must add up all the sides. The details, of course, vary from case to case. For example, if you have an arbitrary triangle, the expression would be something like side1 + side2 + side3 (or whatever the sides are called). In some cases you can simplify the resulting expression; in an isosceles triangle, you have two equal sides, so you get an expression similar to a + a + b = 2a + b. In a rectangle, opposite sides are equal, so you get perimeter = a + b + a + b = 2a + 2b or 2 (a + b).
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Basically, the perimeter of a shape is the total legnth of all it's sides, therefore to write an expression for it, you need to get the legnths of the sides (even if there is a letter (eg: n)), then put them into an expression. For instance: A polygon has 2 sides that are 15cm long, 1 side that is 22cm long, 4 sides that are n long and 3 sides that are a long. To write the expression for the perimeter of this polygon as basically as possible, you would write: 15cm + 15cm + 22cm + n + n + n + n + a + a + a Now you need to simplify it: 52cm + 4n + 3a If you were asked this question, you would write 52cm + 4n + 3a as your answer. That is an expression for the perimeter of that particular polygon. If you apply the same theory to any question like the one you asked about, you would get it right.
How do you write an expression
call width x so length is 3x perimeter is (2*length)+(2*width) so perimeter is 8x
Write an equation for the perimeter, and solve it. Remember that the perimeter is the sum of all four sides.
Represent the length of the rectangle by L and the width by W. The perimeter = 2L + 2W = 2(L + W). The area = L x W.