The two-dimensional topics hereafter are for the most part limited to just five standard figures: triangles, rectangles, squares, trapezoids and circles.  Only the "trapezoid" typically triggers an ominous sense of foreboding as the other four are rather routine, everyday encounters.  Anyway, we will have the opportunity to see that all the other polygons (i.e., the triangle, the square, and the rectangle) are each a special case of the general figure casually known (to the the excessively erudite) as a trapezoid. To begin, we present these five figures, along with a corresponding pair of formulas in order to determine the Perimeter and the Area for each of them.
     The simplest polygon possible will be the 3-sided figure that virtually everyone recognizes as a triangle: The Perimeter (i.e., distance around the polygon) is merely the sum of the three sides a, b, and c.  The Area is a little less obvious, however, it can be found by taking the product of ½ times the (length of the) base times the height of the triangle.  Thus... P  =  a + b + c         (Perimeter)
A  =  0.5 × b × h       (Area)      

     Perimeter & Area formulas for the rectangle involve its two basic dimensions represented most often as the "length" and "width." The Perimeter is again simply the sum of the (four) sides, two of which are length "l" and the other two are of width "w."  The Area is the product of the length times the width.  That is, for a rectangle... P  =  2l + 2w     (Perimeter)
A  =   l × w          (Area)            The square is most likely the easiest case to understand and remember: All four sides are of each of length "s," so four times "s" represents the sum of all of its sides, or