
Trapezoid area and perimeter(Coordinate Geometry)
The area and perimeter of a trapezoid can be calculated if you know the
coordinates of its
vertices.
Try this
Drag any vertex of the trapezoid below. It will remain a trapezoid and the area and perimeter calculated.
You can also drag the origin point at (0,0), or move the rectangle itself.
Area
The area of a trapezoid is calculated by multiplying the average width by the altitude.
See Trapezoid definition (coordinate geometry) to see how the side lengths
and altitude are found. (Note too that the median length is the same as the average width.)
As a formula:

where
b1, b2 are the lengths of the two bases (BC and AD)
a is the altitude of the trapezoid


In the figure above, drag any vertex of the trapezoid and note how the area is calculated.
Perimeter
The perimeter of a trapezoid is simply the sum of all four sides.
Since they have no relationship to each other, there is no formula for it.
Simply find the four side lengths and add them up. The length of each side is found using the techniques described in
Distance between two points (given their coordinates)
which are used to find the distance between each side's endpoints.
Example
Find the area and perimeter of the trapezoid in the figure above. First press 'reset' and 'show altitude'.
Area

First, we need the length of the two bases (the parallel sides).
These are found by calculating the distance between the endpoints of the lines segments.
(See Distance between two points). Doing this we see that
BC = 22 and AD = 47

Then we need the altitude. This is the perpendicular distance between the bases. As described in
Trapezoid (coordinate geometry) there are several ways to do this
depending on whether the trapezoid is rotated or not. Doing this we see that
altitude = 21

Finally we calculate the area as the altitude times the average width ( average base length):
Which agrees with the calculated figure above.
Perimeter

The perimeter is the sum of the four side lengths.
So these are found by calculating the distance between the endpoints of the lines segments.
(See Distance between two points). Doing this we see that
BC = 22 AD = 47 AB=22 CD=28

Finally we add them up to get the perimeter
22 + 22 + 28 + 47 = 119
Which agrees with the calculated figure above.
Rotated case
In the figure above the trapezoid bases are parallel to the xaxis which makes calculations easy.
If you click 'rotated' this will not be the case.
All the techniques described above will still work, but you have to use the correct method for finding the distance between two points,
and the altitude, which requires the correct method for finding the perpendicular distance from a point to a line.
See
Things to try

Click on "hide details" and "rotated" then drag the vertices of the trapezoid around to create an arbitrary size.

From the coordinates of the corner points, calculate the area and perimeter of the trapezoid.

Then click on "show details" to check your result. (The results shown above are rounded off to whole numbers for clarity)
Limitations
In the interest of clarity in the applet above, the coordinates are rounded off to integers and the lengths rounded to one decimal place.
This can cause calculatioons to be slightly off.
For more see
Teaching Notes
Other Coordinate Geometry entries
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