# How to calculate the integral of a function with a spline in it

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Renan Kops on 21 Apr 2017
Commented: Andrew Newell on 25 Apr 2017
Hello Everybody,
So i have a function called FB, and it is the product of a couple of functions
nB = spline(Wavelength,B3);
emi = @(x) 1.989e-6*(10^9*x).^2-0.002;
FB = @(x,T) nB(x).*emi(x)./(x.^5.*(exp((h.*c)./(x.*k.*T))-1))
%integration:
RaBG(Counts) = integral(@(x)FB(x,T),400e-9,720e-9)
nB is a function i must adquire from data. When fitting it with a polinomial function i get a small error which i believe is making me get wrong results, though the code works. I'm trying to fit it with a spline but haven't been able to get the code to work. I also tryied calling FB as this, which didn't work:
FB = @(x,T) ppval(nB,x).*emi(x)./(x.^5.*(exp((h.*c)./(x.*k.*T))-1));
John D'Errico on 25 Apr 2017
I'd like to chime in here,but without knowing what wavelength, B3, and G3 are, it is impossible to give a useful answer.

Andrew Newell on 24 Apr 2017
Edited: Andrew Newell on 24 Apr 2017
For evenly or unevenly spaced data, you could use the trapezoidal rule (MATLAB function trapz).
Interpolation is also reasonable. How exactly isn't the code working?
I don't see any values assigned to c, k or T. Are you doing that earlier? If so, it would make sense to define
FB = @(x) ppval(nB,x).*emi(x)./(x.^5.* (exp((h.*c)./(x.*k.*T))-1));
Andrew Newell on 25 Apr 2017
I'm glad to know that the alternative methods are working!