I'm learning Chain Rule in Multivariable Calculus through James Stewart's book. Everything seemed great in the beginning, until I stumbled upon an example that requires using the Chain Rule twice (second-order derivative). It follows:
From the "But, using the Chain Rule again" forward I just can't grasp what he has done (I'm looking at it for the past two days, I have absolutely no clue, even though I'm familiar with the Chain Rule concept).
Can someone explain that part?