Symbolic evaluation behaving weird

[SS_9781956]

With the four equation evaluated symbolically I am getting results for 3 but the same is giving result as 0 for the last equation any idea?

[Werner_E]

And why do you think thats wrong?

The equation is linear in the first three variables you differentiate to, but constant wrt the last one. So the results seem to be correct.

[SS_9781956]

The value for R.sf and R.h is one and same along with rest of equation one gives the value other is giving zero so it seems wrong 

[LucMeekes · Accepted solution]

Nothing weird.

Did you intend to differentiate the same expression to four different variables?

Then look at these expressions.

The first is different from the last three: it adds R.hcc to a quotient, instead of R.hch.

The 4th differential is to R.h, while R.h is not part of the expression, hence the result is 0.

Success!
Luc