Question $/$ Cade $(2\mathrm{.}1)(3$ Marks$)$:

$1-$ Evaluate the following limit $\underset{(x,y)(10,6)}{lim}\frac{\sqrt[\phantom{2}]{xy}-4}{xy-16}$

$2-$ Find an equation of the plane tangent to the following surface $xyxzyz=12$ at the point $(2,0,6).$