f(x,y)=x^y;
fx=y*x^(y-1);
fy=x^y;
f(1,2)=1;
fx(1,2)=2;
fy(1,2)=1;
f(1.04,2.02)-f(1,2)=fx(1,2)*(1.04-1)+fy(1,2)*(2.02-2)
=2*0.04+1*0.02
=0.1;
then f(1.04,2.02)=f(1,2)+0.1
=1+0.1
=1.1;