Area vs Perimeter of a triangle
Drag the orange dot on the triangle below. The triangle will have a fixed
, but the
A common error is to assume that a triangle that has a fixed
must also have a fixed
This is definitely not the case as can be seen from the figure above. As you drag the orange point A,
the triangle will maintain a fixed perimeter. But as you can see, the area varies quite a bit.
When A is half way between B and C, the area is at a maximum. As you drag it around towards one side you can see the
area decreasing, both in the formula at the top and by noticing that fewer and fewer squares can fit inside it.
Eventually, when A is in line with B and C, the area is zero.
The area is at a maximum when the triangle is
That is, when both sides have the same length. Carefully adjust A above
to create an isosceles triangle and note the area is the greatest when AC and AB are both the same length (9.0)
Try it with string
Make a loop of string and pass it around two pins (corresponding to the two points B and C above).
Pull the string taut with a third pin to make a triangle. As you move any pin with the string tight,
you will be making triangles with different areas but the perimeter is fixed (the length of the string loop).
The Ellipse Connection
In the figure above, select the "Show trail" checkbox, then drag point A all the way around the base line.
The resulting shape is an
Why is this? The definition of an ellipse is
"A line forming a closed loop, where the sum of the distances from two points (foci) to every point on the line is constant"
Points B and C form the two foci. Since the distance from B to C is fixed, and the perimeter is fixed, then the sum of the distances AB and AC are
constant - the condition required to form an ellipse.
Learn more about Ellipses
The string experiment described above is actually a practical way to draw an ellipse.
See Drawing an ellipse using string and 2 pins. For more on ellipses, see also
Definition of an ellipse.
Related triangle topics
Perimeter / Area
Congruence and Similarity
Triangle quizzes and exercises
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