1. The problem statement, all variables and given/known data Ok this isn't really homework just something I came across and am confused by. I came across a function that looks similar to z = xy and I found that delta_z / delta_x = dz/dx which is really weird to me. This was really strange to me so I checked to see if z = x²y would also be the same and it wasn't which I expected. I don't know how to explain why z = xy has this sort of property where delta_z / delta_x = dz/dx. Sorry if this is a sort of weird question. 2. Relevant equations For example let's say z = xy, x = 5, y = 2, z = 10. We increase x to 6 then z = 12. delta_z / delta_x = dz/dx = y => delta_z = y*delta_x = 2*1 = 2. No idea why this is true though it should be like a linear approximation. 3. The attempt at a solution So I was thinking it's true in the same way that for y=mx+b, delta_y/delta_x = dy/dx, now I don't know what to call these type of functions.