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I was wrong in the comments, since you have an analytical expression for θ, there is no need for an InterpolationFunction. f[y_, θ_] = 2 y Cos[θ] + 4 y^2 (1 - 3 Cos[θ]); gradf[y_, θ_] = Grad[f[y, θ], {y, θ}]; v0 = f[y, θ]/gradf[y, θ]; There was a typo, the Cos didn't have brackets in the first line, also I added v0 to save the evaluated expression before ...


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