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Rule

Area of a Rectangle

The area of a rectangle is calculated by multiplying the rectangle's length, times its width,

The length is usually defined as the rectangle's longer sides and the width its shorter sides, but this assignment is arbitrary.

Rectangle of length l and width w

Proof

Consider a square of unit side length. In this case, by the definition of area, its surface would be a squared unit. Now, for a rectangle of integer length and integer width it is possible to divide it unit squares.
Rectangle divided in unit squares
Since the original rectangle has side length and width there are exactly unit squares in each row and there are rows of unit squares. Therefore, the total number of unit squares is Hence, the area of a rectangle of side length and width is This result is still valid if and were any real numbers.