$15,16,17,18,19,$ and $20$ Determine whether or not the vector field is conservative. If it is conservative, find a function $f$ such that $F=$gradf.

$15.F(x,y,z)=($:$2x{y}^3{z}^2,3x^2{y}^2{z}^2,2x^2{y}^{3}z$:$)$

$16.F(x,y,z)=($:$yz,xz+y,xy-x$:$)$