ｘ：独立変数⇒ｄｘ：無限小⇒ｄｄｘ＝０；always
ｘ：従属変数⇒ｄｘ：無限小⇒ｄｄｘ≠０；sometimes
ｘ：独立変数⇒⊿(ｄｘ)＝ｄ(⊿ｘ);always
ｘ：従属変数⇒⊿(ｄｘ)＝ｄ(⊿ｘ);sometimes