How does drawing a rectangle support the idea of composing a larger unit from smaller units

When you think rectangles, you think areas. Small areas aggregate to form bigger ones.

If you drew a rectangle, it's easy to divide it into smaller units by simply taking a line through its midsection from one length to the other and one width to the other. One can either form smaller squares or rectangles from a larger one.

When that is one, you'd have taken the larger area of a rectangle which is simply the product of the value of its length and it's the breadth, and divided it into smaller units.

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