Dr为r的微元,o(Dr)为Dr的高阶无穷小
s(r+Dr)=π(r+Dr)²=π(r²+2r*Dr+Dr²)=π(r²+2r*Dr+o(Dr))
s(r+Dr)-s(r)=π(2r*Dr+o(Dr))=2πr*Dr
s'(r)=[s(r+Dr)-s(r)]/(r+Dr-r)=2πr*Dr/Dr= 2πr