course Mth 275 Section 12.6 I found this section interesting. It makes sense as an extension of the interpretations of derivatives of a single variable. I had a problem with only one of the questions, #57: which gives a formula for Newton’s law of universal gravitation and says to show that d/dx (1/r) = -x/r^3, and similarly for d/dy and d/dz, given that r^2=x^2+y^2+z^2. I was unable to get anywhere close to this using partial differentiation. I had no problem with the proofs in the other problems.