



When we
have an object with four sides



but only
two of the sides are parallel 


we call
it a TRAPEZOID. 


(Impressive
name, eh?) 











Most of
the time, the sides of a trapezoid 


are all
different lengths. 


Sometimes
the left side and right side 


turn out
to be the same length. 





To
calculate the area of a trapezoid, 


we do
something like what we did with the rhombus. 


Here, we
cut off a piece from each end. 


Like this
... 











and flip
them up to the top of the drawing. 


Like this
... 











Now we
have a rectangle, 


and that we can
deal with. 











Another
way to look at this, 


is that
we chopped off some of the length 


of the
long side on the bottom. 


And add
those pieces to the short side on the top. 











So that
both the top and bottom lengths are the same. 











We can do
that with two numbers too! 


We just
add them together and divide by two. 


We call
that finding the average. 





Example: 


Make the
top and bottom sides of the trapezoid 


the same
length and find that length. 











So great,
now the top and bottom sides 


are the
same length. 


But we
still can't figure out the area of the trapezoid. 


We need
to know how far it is 


from the
bottom to the top 


(just
like with the rhombus) 


That
measure is called the height or altitude. 


Say the
height of this trapezoid was 4. 


We would
get: 











Let's do
one more ... 





Example: 


Find the
area of the trapezoid ... 











1) Add
the top and bottom sides together 


and
divide by two. That is, find the average ... 











2)
Multiply that number times the height. 





Area
= 6 ×
7



Area = 42






copyright 2005 Bruce Kirkpatrick 
