Dear All, I am unable to understand a simple mathematics relation. I spent 2-3 hours to Google multi-variable mathematics and have studied some concepts, still i am missing/confusing some basics. The problem I have at hand is following. Vector p can be written as p = (p1, p2, p3) = n(sin θ3 cos φ, sin θ3 sin φ, cos θ3) As defined earlier, p1, p2, and p3 are the x1, x2, and x3 components of vector p and therefore dp1dp2 can be written as shown in attached figure. Kindly help me understand the multiplication of these two total differentials dp1*dp2. I cannot find any easy reference to product of total differentials. I will appreciate reference to any material. Many thanks for your help and support. Regards, Taimoor.