Start with a square of side length and add two rectangles, each with side lengths and . The total area is . Place the rectangles along two adjacent sides of the square: the missing corner is a by square, of area . Filling it produces a square of side length .
The area picture explains why the half-coefficient is squared. It also separates two operations that are easy to confuse: adding changes , whereas adding and subtracting preserves the expression. When solving an equation instead, adding to one side requires adding to the other side too.
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