diff --git a/Doc/library/decimal.rst b/Doc/library/decimal.rst
index d0dedda..d1e8bc7 100644
--- a/Doc/library/decimal.rst
+++ b/Doc/library/decimal.rst
@@ -1717,6 +1717,9 @@
    def cos(x):
        """Return the cosine of x as measured in radians.
 
+       The taylor series approximation works best for a small value of x.
+       For larger values, first compute x = x % (2 * pi).
+
        >>> print(cos(Decimal('0.5')))
        0.8775825618903727161162815826
        >>> print(cos(0.5))
@@ -1740,6 +1743,9 @@
    def sin(x):
        """Return the sine of x as measured in radians.
 
+       The taylor series approximation works best for a small value of x.
+       For larger values, first compute x = x % (2 * pi).
+
        >>> print(sin(Decimal('0.5')))
        0.4794255386042030002732879352
        >>> print(sin(0.5))