diff --git a/source/calculus/notations.rst b/source/calculus/notations.rst
index 6a8ac2e..b603eab 100644
--- a/source/calculus/notations.rst
+++ b/source/calculus/notations.rst
@@ -12,6 +12,10 @@ The Leibniz's notation expresses the derivative of :math:`f` as :math:`\frac{dy}
 
 More details `here <https://www.me.psu.edu/cimbala/me420/Homework/dydx_quotient_article.html>`__.
 
+.. note::
+   The following :math:`\frac{dx}{dt}` means :math:`x` is a function of :math:`t` such as :math:`x=f(t)`.
+   See explanations `here <https://www.khanacademy.org/math/ap-calculus-ab/ab-diff-contextual-applications-new/ab-4-4/v/differentiating-related-functions-intro>`__.
+
 Lagrange's Notation
 -------------------
 Also cal prime notation