Write a math question for the theme "Characteristics of constant function f(x)=c": Find the value of c if the constant function f(x)=c passes through the point (5, 9).
+
Find the equation of an ellipse centered at (h, k), with a major axis length 2a and a minor axis length 2b.
+
What is the smallest number of the form 2^n + 1 that is divisible by 641? (