A 48 by 18 rectangle is filled with the largest possible squares in stages: two 18-squares leave a 12 by 18 strip, one 12-square leaves a 12 by 6 strip, and two 6-squares complete the tiling exactly. Each tiling stage corresponds to one division step in Euclid's algorithm, and the final square size, 6, is the greatest common divisor. Useful for students learning why the algorithm works geometrically, not just as arithmetic steps.
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To find gcd(48, 18), tile a 48 by 18 rectangle with the largest squares possible. Two 18-squares leave a 12 by 18 strip; one 12-square leaves a 12 by 6 strip; two 6-squares finish it exactly. Each step is one line of Euclid's algorithm, and the last square, 6, is the gcd.