x = .5;
h1 = .25;
f = [25,-6,+7,-88];
fp = [75,-12,+7];
fpp = [150,-12];
X1 = [x-h1,x,x+h1];
fX1 = polyval(fp,X1);
cd1 = (fX1(3)-fX1(1))/(2*h1)
fPx1 = polyval(fpp,x);
x = .5;
h2 = .25;
f = [25,-6,+7,-88];
fp = [75,-12,+7];
fpp = [150,-12];
X2 = [x-h2,x,x+h2];
fX2 = polyval(fp,X2);
cd2 = (fX2(3)-fX2(1))/(2*h2)
fPx2 = polyval(fpp,x);

etc1 = abs((fPx1 - cd1)*100/fPx1)
etc2 = abs((fPx2 - cd2)*100/fPx2)