Basic Math and Pre-Algebra For Dummies (79 page)

To measure the area of a rhombus, you need both the length of the side and the height. Here's the formula:

So here's how you determine the area of a rhombus with a side of 4 cm and a height of 2 cm:

You can read 8 cm
²
as “8 square centimeters” or, less commonly, as “8 centimeters squared.”

The top and bottom sides of a parallelogram are called its
bases
(
b
for short), and the remaining two sides are its
sides (s)
. And as with rhombuses, another important measurement of a parallelogram is its
height (h)
, the shortest distance between the bases. So the parallelogram in Figure 
16-13
has these measurements:
b
= 6 in.,
s
= 3 in., and
h
= 2 in.

Each parallelogram has two equal bases and two equal sides. Therefore, here's the formula for the perimeter of a parallelogram:

To figure out the perimeter of the parallelogram in this section, just substitute the measurements for the bases and sides:

And here's the formula for the area of a parallelogram:

Here's how you calculate the area of the same parallelogram:

The parallel sides of a trapezoid are called its
bases
. Because these bases are different lengths, you can call them
b
₁
and
b
₂
. The height
(h)
of a trapezoid is the shortest distance between the bases. Thus, the trapezoid in Figure 
16-14
has these measurements:
b
₁
= 2 in.,
b
₂
= 3 in., and
h
= 2 in.

Because a trapezoid can have sides of four different lengths, you really don't have a special formula for finding the perimeter of a trapezoid. Just add up the lengths of its sides, and you get your answer.

Here's the formula for the area of a trapezoid:

So here's how to find the area of the pictured trapezoid:

In this section, I discuss how to measure the perimeter and area of all triangles. Then I show you a special feature of right triangles that allows you to measure them more easily.

Mathematicians have no special formula for finding the perimeter of a triangle — they just add up the lengths of the sides.

To find the area of a triangle, you need to know the length of one side — the base (
b
for short) — and the height
(h).
Note that the height forms a right angle with the base. Figure 
16-15
shows a triangle with a base of 5 cm and a height of 2 cm: