Hi, I was talking to a friend about the chain rule i.e. dy/dx =dy/du X du/dx. She said that technically it is incorrect to say that this works because the 'du's cancel each other out. I didnt quite understand the explanation. Can anyone else explain to me? Also if you have for example dy/dx =1 , you can solve this as a differential equation by seperating the variables so you have integ 1 dy = integ 1 dx, but why can you seperate the variables? How does that make sense, isnt dy/dx itself as a whole the gradient.