You need to limit the range of x and y that are fed to trapz, you could do something like this, assuming your original variables are called x and y (modify accordingly for your actual variable names)
% assume x and y are defined earlier
% define range of interest
xRange = [21,31.3]
% find logical indices for range of interest
idl = x >= xRange(1) & x <= xRange(2);
% provide just values of interest to trapz
A = trapz(x(idl),y(idl))
