1. To find the perimeter of a rectangle, just add up all the lengths of the sides: Perimeter = L + w + L + w         = 2L + 2w
To find the area of a rectangle, just multiply the length times the width: Area = L x w
Rectangle
A rectangle is a four-sided polygon (a flat shape with straight sides) where every angle is a right angle (90°). Also opposite sides are parallel and of equal length. Example: A square is a special type of rectangle.

2. All sides are the same length (congruent) and all interior angles are the same size (congruent). To find the measure of the interior angles, we know that the sum of all the angles is 180 degrees (from above)...  And there are three angles...
So, the measure of the interior angle of an equilateral triangle is 60 degrees.
To find the measure of the central angle of an equilateral triangle, make a circle in the middle...  A circle is 360 degrees around...  Divide that by three angles...
So, the measure of the central angle of an equilateral triangle is 120 degrees.
Triangle
Triangles, as the prefix tri suggests, are closed geometrical figures that have three straight sides. Every triangle will, as a result, have three angles as well.

3. Parallelogram
Opposite sides are parallel and equal in length, and opposite angles are equal (angles "a" are the same, and angles "b" are the same)
To find the perimeter of a parallelogram, just add up all the lengths of the sides: Perimeter = a + b + a + b = 2a + 2b
To find the area of a parallelogram, just multiply the base length (b) times the height (h): Area = b x h

4. Circle
The circumference of a circle is the perimeter -- the distance around the outer edge. Circumference = where r = the radius of the circle and pi =
Area = where r = the radius of the circle and pi =
The set of all points on a plane that are a fixed distance from a center.

5. Irregular
Example: For the shape highlighted above, we take the two heights (the "y" coordinates 2.28 and 4.71) and work out the average height:
( )/2 = 3.495
Work out the width (the difference between the "x" coordinates 2.66 and 0.72)
= 1.94
The area under that line segment is width×height
1.94 × =
A polygon that does not have all sides equal and all angles equal. A polygon is "regular" only if all angles are equal and all sides are equal otherwise it is irregular