Hi Alan and others,
I have only just come back to this question to take a proper look.
Prior to watching you one the forum I never treated calculus dy and dx so much like normal algebra.
To me dy/dx was glued together with super glue.
I guess I understood better when I was studying at university but I do not remember that.
It all makes such perfect sense when you show me how to do things.
Working through this with you and WolframAlpha as a guide made it so easy.
BUT, if I come back in a fortninght and try to do it myself I won't be able to.
Isn't that sad. :)
Students often tell me that I do not understand their learning problems, but you see I really do!
You might prefer the following Melody:
Strictly, dy and dx should be glued together, but we science/engineering types often take liberties with the mathematics!!