A cone with a base of radius r and a height h will have a volume of Pi*h*(r^2)/3, and a surface area (minus the base) of r*Pi*sqrt(r^2+h^2).
To set up an equation to do the Lagrange multiplier method for _all_ boundaries, you may wish to type
linalg[grad](volume + a*surface area of cone + b*((s/r)-1.5),
Set the three coordinate functions equal to zero, and solve them with the surface area = 84 and s=1.5*r.
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