1. In the Khan academy video I watched on partial derivatives, I understand absolutely everything except for the last 20 seconds which confused me. http://www.youtube.com/watch?v=1CMDS4-PKKQ Using the formula: Z = x² + xy + y² @z/@x = 2x +y x=0.2, y=0.3 2(.2) + .3 = .7 What I know this means: At an x value of 0.2 and a y constant of 0.3, the slope of the tangent line will be 0.7. At the end of the Khan video though, he stated: "So everytime X increases, 1, Z will increase by 0.7" Did I miss something or is this a mistake? What is in the video is a curve that is constantly changing so the slope will always be different. The only way to say that every increase in X will result in 0.7 is if we are dealing with a planar surface correct?