Suppose D = Ax^{a_{1}}_{1} x^{a_{2}}_{2}· · · x^{a_{n}}_{n} is defined for all x_{1} > 0, x_{2} > 0, . . . , x_{n} > 0, where A > 0 and a_{1}, a_{2}, . . . , a_{n} are constants. Find the elasticity of D w.r.t. x_{i}, for i = 1, . . . , n.
Because all the factors except x^{a_{i}}_{i} are constant, we can apply Eq. (7.7.3) to obtain the result El_{i} D = a_{i}.
f(x)=A x^{b}~\Rightarrow~\mathrm{El}_{x}\,f(x)=b (7.7.3)