Not able to solve a differentiation

Hello all I am trying to use fsolve to solve multiple equations. In one of the equation I am using differentiation with respect to the variable I am solving for for every previous time step. I am getting an error saying Difference N must be a scalar positive integer. I tried using sym but I am not sure how I should proceed with it. I have attached the file with associated text kindly have a look and help me figuring this out.

Answer 1

[Walter Roberson]

Your code appears to be attempting to differentiate with respect to a numeric value.

syms Tgc1
pq_sym = simplify( diff(0.622*610.78*Rh*exp(a*(Tgc1-273.16)/(Tgc1-b))/(Pa - 610.78*Rh*exp(a*(Tgc1-273.16)/(Tgc1-b))),Tgc1) );
pq = double( subs(pq_sym, Tgc1, Tg(c-1,1)) );