f(x,y)=x^y
dz=yx^(y-1)dx+x^ylnxdy
取x=1,dx=-0.01,y=2,dy=-0.02
dz=yx^(y-1)dx+x^ylnxdy
=-0.02-0.02ln2=-0.02(1+ln2)= -0.0339
0.99^(1.98)=1-0.0339=0.9661 (准确值为0.9803)