A=[6 8 8; 21 6 3; 18 8 6]
using Symbolics
@variables x,y
toy(i)=7-2*i
tox(j)=2*j-1
R=[((x-tox(j))^2+(y-toy(i))^2)^0.5 for i=1:3,j=1:3]
z=3.2+1.7*sum(A.*R.^0.91)/84
fs="fz(x,y)="*string(z)
eval(Meta.parse(fs))
using Plots
contour(0:0.05:6,0:0.05:6,fz)
gradf=Symbolics.gradient(fz(x,y),[x,y])
gs="g(x,y)="*string(Symbolics.toexpr(gradf))
eval(Meta.parse(gs))
jacg=Symbolics.jacobian(g(x,y),[x,y])
Dgs="Dg(x,y)="*string(Symbolics.toexpr(jacg))
eval(Meta.parse(Dgs))

xn=[2.0,3.0]
xn=xn-Dg(xn...)\g(xn...)
xn=xn-Dg(xn...)\g(xn...)
xn=xn-Dg(xn...)\g(xn...)
xn=xn-Dg(xn...)\g(xn...)
xn=xn-Dg(xn...)\g(xn...)

scatter!((xn[1],xn[2]))