They all get the perimeter of the circle correct but only Approx 2 and 3 and Series 2 get close to the value of 40 for the extreme case of b0. The Perimeter of the square formula is given by Perimeter Sum of the lengths of 4 sides. What this formula means in simpler terms is that to find the perimeter of a triangle you just add together the lengths of each of its 3 sides.
In a rectangle the opposite sides of a rectangle are equal and so the perimeter will be twice the width of the rectangle plus twice the height of the rectangle.
This is the equivalent of adding all four sides since opposite sides are of equal length by definition. Perimeter has a Greek origin peri means around and meter means measure. In the case of a circle the perimeter is called a circumference. Be sure to only add similar units.