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5 steps: (1) CIRCUMFERENCE = distance around a circle (its perimeter). (2) C = π × d (pi times diameter) or C = 2πr (2 × pi × radius). (3) π ≈ 3.14159... (use 3.14 or the π button). (4) Example: r=7cm. C = 2π(7) = 14π ≈ 43.96cm. (5) Working backwards: if C=31.4cm, then d=C/π=31.4/3.14=10cm, so r=5cm

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