The area of a rectangle is often defined to be the product of its length times its width.
Making the area of the rectangle a definition rather than a theorem which would have to be proved from presumably more basic assumptions seems right to me.
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... right angles, by definition of a rectangle.... right angles, by definition of a rectangle.... right angles, by definition of a rectangle.... right angles, by definition of a rectangle.
It's half the area of a rectangle
Area of a rectangle = length x width
Area of a rectangle: a = l * w
Area of a square/rectangle=its length × its width