D(f(x))=f'(x)D^2(f(x))=f''(x)(1+D^2)y=1/cosx(1+iD)(1-iD)y=1/cosxy=[1+(iD)+(iD)^2+(iD)^3+.][1+(-iD)+(-iD)^2+(-iD)^3+.](1/cosx)=[1+iD-D^2-iD^3+D^4.][1-iD-D^2+iD^3+D^4.](1/cosx)=[1-D^2+D^4.](1/cosx)在mat...