Linear approximation
a. Find the linear approximation to the function $f$ at the given point.
b. Use part (a) to estimate the given function value.
$$\begin{aligned}
&f(x, y, z)=\ln (1+x+y+2 z) ;(0,0,0) ; \text { estimate }\\
&f(0.1,-0.2,0.2)
\end{aligned}$$