 A formula is a term used in the computation of values by hand or machine. Although some attributes of physical objects lend themselves to direct measurement such as length and mass, others such as area, volume, and density do not and must be computed from more readily observed values with the help of the appropriate formula. For example the area of a rectangle \(L\) inches long by \(W\) inches wide is given by the formula \(LW\) in units of square inches, the volume of a ball of radius \(r\) is \(4\pi r^3 /3\), and the density of a solid of mass \(M\) and volume \(V\) is given by \(M/V\).