How to derive and integrate this interpolation function?

I have a problem with this function that I interpolated with a discrete data, I need to derive and then integrate to have a numerical values.

perfil = Import[data,"Table"]     Extrados = Table[perfil[[x]], {x, 1, 26}]; Intrados = Table[perfil[[x]], {x, 26, 51}]; \[Eta]u = Interpolation[Extrados]; \[Eta]i = Interpolation[Intrados]; \[Eta]c = 1/2 (\[Eta]u[x] + \[Eta]i[x]); d\[Eta]c = D[\[Eta]c, x] 

But I have the following output: So I can’t have a numerical values. I need to derive it and then integrate it

Subscript[B, 0] = 2/\[Pi] \!\( \*SubsuperscriptBox[\(\[Integral]\), \(0\), \(\[Pi]\)]\(d\[Eta]c \ \[DifferentialD]\[Theta]\)\) 

But the solution it’s the same. Someone know another solution about this? Why mathematica doesn’t gave me a symbolic function?