Base of a plane figure - Definition and illustration.

The base of a plane figure is the side on which the figure  " sits ." You could also say that it is the bottom of the plane figure.

We show the base with a red segment for the trapezoid and the triangle below.

Base of a plane figure

Notice how the base of the trapezoid in the figure above happens to be the shortest side in this situation.

The base of a plane figure does not have to be the shortest side or the longest side. Just rotate the figure and make it "sit" on a different side and that side is your base.

After rotating the triangle and the trapezoid, the red segments are no longer the bases for these figures.

Base of a triangle and a trapezoid

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