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Three lengths, laid in a row. That is the side.
Not a formula yet. A stick made of three shorter sticks.
Call them a, b, and c — just names for three lengths. Lay them end to end. The long stick is a plus b plus c. That is all. The rest of the picture is this stick, copied upward, until it becomes a floor.
Drag the three lengths. Watch the row change. Nothing is squared yet.