julia> using ModelingToolkit; @variables x y;
julia> f(x,y)= sqrt((x^2+144)+(y^2+144))
f (generic function with 1 method)
julia> let ex = Differential(x)(f(x, y))
func_ex = build_function(expand_derivatives(ex), x, y)
@eval ∂xf(x, y) = ($func_ex)(x,y)
end
∂xf (generic function with 1 method)
julia> using Plots, ImplicitEquations
julia> plot(∂xf ⩵ 0.1, xlims=(0, 10), ylims=(-100, 100))
produces
In the future, people will have an easier time helping you if you actually show the code that produces the error. It’s a lot harder to help if we have to guess what you’re actually doing.