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The "units" seem to be inconsistent in this formula at the end of the derivation:
The numerator is subtracting from Suppose the values of P are distances measured in meters. Then has units of distance (meters) and has units of area (meters^2). One cannot subtract a distance from an area. Is there a typo in the given formula? - Ac44ck (talk) 04:37, 26 December 2008 (UTC)[reply]