Trapz not referred to x-axis

Hi, I'm having some issues regarding the chance to calculate the area between two curves or lines, as the ones following:

clear all
close all
xA=0:4;
A=[-2 -1 2 3 4];
figure(1)
plot(xA,A,'o');
grid on
hold on
%polyfit to build a curve with "A" data
[pA,~,muA] = polyfit(xA, A, 4);
fA = polyval(pA,xA,[],muA);
plot(xA,fA);

 xi = linspace(min(xA), max(xA));            % Interpolation Vector
 Ai = interp1(xA,A,xi,'linear');             % Interpolated Data
 y=xi*(-1)-2;
 plot(xi,y);
 Areabetweencurve=trapz(abs(y),abs(Ai));    % area between the curves should be around 21

I thought that trapz could have been the right tool, but instead of the x-line as a reference, trapezoids should be considered between the two lines. The point is, I could calculate the area between the two curves with a loop where I consider (b+B)*h/2, but maybe (and probably) there is a smarter way. Any suggestion? thanks

[Scott MacKenzie]

Check your trapz arguments. For the 2-arg version, the first argument is for the x spacing.

Answer 1

[Cris LaPierre]

If you want to use trapz, use it to calculate the area below both lines, and then subtract them to find the area between the lines.

[Simone Speltoni]

Thank you Cris