Two surfaces are said to be orthogonal at a point P of intersection if their normal
Chapter 9, Problem 38(choose chapter or problem)
Two surfaces are said to be orthogonal at a point P of intersection if their normal lines at P are orthogonal. Prove that the surfaces given by F(x, y, z) = 0 and G(x, y, z) = 0 are orthogonal at P if and only if FxGx + FyGy + F,Gz = 0.
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