Blank screen when plotting a graph and how to partial differentiate?

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p_RK = @(v,T) (R*T*(1/(v-b) - a/(R*T^1.5*v*(v+b)))) % The redlich kwong equation
How can I partiall differentiate this function with respect to v?
Also plotting this seems to work for the pressure coefficient
alphav = @(v) ((R/(v-b)+a/(2*T^1.5*v*(v+b)))/p_RK_T(v))
% plot alpha_v over the required range
figure
fplot(alphav, [v1 v2])
hold on
but when I try plotting this I get a blank screen:
diffp_volume=@(v) (((-R*T)/(v^2-b))+(a*(2*v+b)/(v^2*T^1/2*(v+b)^2)))
kt = @(v) -1/diffp_volume*v
% plot alpha_p over the required range
figure
fplot(kt, [v1 v2])
hold on
What can I do? Also I can send the whole code if this doesnt make sense
Thanks
As a note this is about thermodynamics on plotting the coefficient of thermal expansion

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