First you would want to graph the rectangle. For example, The corners of the rectangle are (0,0), (0,2), (3,0) and (3,2). You would have a rectangle with the vertical sides being 2 units in length and the horizontal sides being 3 units.
The "easy" way to find the area of a rectangle is to multiple the length of the vertical sides by the horizontal sides. In this example, 2*3=6. The calculus way would be to setup an integral from a to b of f(x)dx. a and b are the end points for x values. i.e. a <= x <= b. In this example, a = 0 and b = 3. f(x) is the function y=2. The integral from 0 to 3 of 2dx = 6.
The formula for the area of a rectangle is length x breadth. In order to prove this works, work out an area of a rectangle using that formula.
break it up into parts (i.e. Pythagorean theorem plus basic area) or learn/use calculus.
newton
Calculus was created to prove physics which defines the laws of nature.
By using Differential Calculus. Any rectangle is at a maximum area when it is a square. So taking 108 and dividing by '4' We have '27' This is the length of one side of the square So its areis A(sq) = 27^2 = 729 m^2
Issac Newton
Rectangle area = (rectangle width) x (rectangle height)
From Wikipedia: "...a rectangle is any quadrilateral with four right angles". So there isn't much to prove, that's how the rectangle is defined.
The link has the answer to your question. http://www.sosmath.com/calculus/integ/integ03/integ03.html
A = lw Area of a rectangle = length times width
Use squares and try it out for yourself. Get a number of squares and make a rectangle 3 squares long by 4 squares wide. Count the squares. You should have 12 squares (or 3*4). That's the best way I know to prove the formula.
Perimeter is a unit of length. Area is a unit of area. The two units are not directly convertible.However, the area of a rectangle is length times width, and the perimeter is two times length plus two times width. Given constant perimeter, a square has maximum area, while a very thin rectangle has nearly zero area. (In calculus terms, the limit of the area as length or width goes to zero is zero.)Depending on how you want to name your units, you can always find a rectangle whose perimeter is "larger" than area, but this is a numerical trick that is not valid in any school of thought of mathematics that I know.