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At a Glance - Four-Sided Shapes

Don't you love it when you can rely on common sense rather than having to memorize complicated formulas? Yeah, us too. Well then, you'll absolutely adore four-sided shapes. You might even marry them. You and four-sided shapes, sitting in a tree, K-I-S-S-I-N-G.

To determine the distance all the way around a four-sided shape, you simply add the lengths of the four sides. If you remember this step, you can figure out whatever formula you need by drawing a picture of the shape and thinking for 30 seconds. If that makes your brain throb, you can think for 15 seconds, take a quick snack break, then come back and think for 15 more.

A parallelogram is a four-sided figure in which opposite sides are parallel. Opposite sides will be equal, and opposite angles will be equal as well. Equality reigns supreme here in Paralleloland. Man, that's a lot of L's.

The perimeter is the sum of all four sides. We could write this as the formula P = 2b + 2where P stands for perimeter. However, it's more important (and simpler) to remember that the perimeter is the sum of all four sides than to commit this formula to memory. You've got enough numbers and symbols floating around in that noggin of yours. Let's free some space for a change.

The area of a parallelogram is its height times its base. To see why that is, chop off the left corner of the parallelogram (watch your fingers):

Then move it over to the right side:

Now we have a rectangle that has the same area as the parallelogram. The area of this rectangle is bh, and therefore, the area of the parallelogram is also bh. What a copycat. If we let be the area of the parallelogram, we can express this fact via the formula:

A = bh

A rectangle is just a particular type of parallelogram: it has right angles at all four corners. Since a rectangle is a parallelogram, opposite sides will still have the same length. Note that this fact doesn't work in reverse—not all parallelograms are necessarily rectangles. Some of them are Virgos.

The perimeter P of a rectangle is given by this formula:

P = 2l + 2w

And the area A of a rectangle is given by this formula:

A = lw

square is a rectangle with all four sides the same length. Squares live in very strict, cookie-cutter communities. Also, they're restricted in the number of guests they can have in their backyard on weeknights.

The perimeter P of a square is:

P = 4s

A square's area is:

A = s2

A trapezoid is a four-sided figure with one pair of opposite sides parallel. The sides that are parallel to each other are called the bases of the trapezoid. That may be a little confusing, since we're used to thinking of a base as being only on the bottom of something, but this feller does indeed have a base on top as well. He's got all his bases covered.

In the picture below, b1 and b2 are the bases.

As with a square, the perimeter is the sum of all four sides. While we could write a formula for perimeter in terms of the names of the sides, it wouldn't make any more sense than writing "the perimeter is the sum of all four sides," so let's not go there. Formulas are cool and all, but straightforward concepts expressed in plain English are the bomb.

Here's the area A of the trapezoid:

To see the justification for this formula, read about the distributive property and simplifying expressions. However, since that'll involve a lot of clicking and reading, our hunch is that you'll assume we know what we're talking about. That's perfectly fine, too.


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