A circle has a radius of 4.5 centimetres. What is its circumference? Give the answer in terms of π.

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Doubling 4.5 gives a full width of 9, and π multiplied by that width measures the way round. Leaving 4.5 as it stands treats a half-width as though it were the whole, while squaring it reaches for the rule that measures the space inside rather than the boundary.