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Show that each of the following functions is concave. In each case the domain consists of all $\left(x_{1}, x_{2} ight)$ such that $x_{1} \geq 0$ and $x_{2} \geq 0$. (a) $f\left(x_{1}, x_{2} ight)=x_{1}^{\frac{1}{4}} x_{2}^{\frac{1}{8}}$ (b) sf\left(x_{1}, x_{2} ight)=\left(x_{1}^{\frac{1}{4}} x_{2}^{\frac{1} {8}} ight)^{\frac{1}{2}}$ (c) $f\left(x_{1}, x_{2} ight)=\ln \left(x_{1}^{\frac{1}{4}} X_{2}^{\frac{13} ight) SCS.SD. 172||