derive with fft and ifft

clear all

clc

z=50;

x=linspace(0,2*pi,z);

a=cos(x);

% derive while using ifft

k=1./x;

tf_a=1i.*k.*fft(a);

da=ifft(tf_a);

plot(x,a,'r',x,da,'b',x,gradient(a)./gradient(x))

legend('cos(x)','ifft(ik(fft(cos(x))))','gradient');

% when I run the program, the amplitude of "da" is different from the amplitude of " gradient(a). /gradient(x)" and it depends on z.

how can I fix this in a way "da "=" gradient(a). /gradient(x)"

thank you in advance