Our very last standard 2-dimensional figure to consider is in so very many respects the most fascinating. That would be the circle — defined as the set of all points "equidistant" from a fixed point referred to as the center (point C in the illustration). The distance from the Center (point C) to any point on the circle is the length of the radius, "r."  The distance around a circle is called the Circumference (rather than Perimeter, although the two ideas are basically the same).  Both this quantity and the Area involve using the rather mysterious number π (i.e., the lower case Greek letter, pi*).
     For a circle, the two important formulas are:

C  =	2πr	&	A  =	πr2
or	πd   where   d = diameter

The diameter of a circle is any line segment with both of its endpoints on the circle and that passes

through the circle's center (point C in the figure). It should be noted that the length of the diameter is twice the length of the radius (i.e., d = 2r) and/or that the length of the radius is one-half the length of  the diameter (i.e., r =12d). 

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     Having now rummaged through the essence of both the Perimeter and the Area, for five common (2-dimensional) figures, one should be adequately equipped at this point to deal with an assortment of 2-D figures that involve finding the distance around that figure (using the Perimeter & Circumference formulas) and/or the amount of space inside the figure (using one or more of the various Area formulas).  * * *      Let's see how we will fare with a few quick examples, beginning with the Perimeter and the Area of an obtuse triangle...