A key point understanding single variable chain rule:

f(g(x)) = f’(g)*g’

At f’(g) part, g is inserted into f’ as a macro ==after== taking the derivative!!!!!

e.g:

f(x)=a^2
g(x)=2a

f'(g), 
instead of dx((2a)^2) /dy
is actually: "dx(a^2)/dy".replace("a","2a") !!!

2017/9/12 20:30