The equation d=2r is used in geometry to find the diameter of a circle when its radius is know. d represents, and r represents the radius. This formula is explained by the fact that the diameter of a circle is equal to 2 times the length of its radius. For example, if we have a circle with a radius of 4, and we need to find its diameter we can use the formula d = 2r: d = 2r = 2*4 = 8
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