Adding more explanations to exercise

Author: HumphreyYang

lectures/functions.md

@@ -368,15 +368,17 @@ write our own version as an exercise.
 for any positive integer $n$.
 
 2. In addition, try to add a new argument for your function.
-The argument will take in a lambda expression f that transforms n to $f(n) = n^2 + 1$ if n is even, and $f(n) = n^2$ if n is odd. 
-The default value will be $f(n) = n$.
+The argument takes a function `f` that transforms n to $f(n) = n^2 + 1$ if n is even, and $f(n) = n^2$ if n is odd. 
+The default value should be $f(n) = n$.
 
 For example
 
 - The default case `factorial(3)` should return $3!$
 - `factorial(3,f)` should return $9!$
 - `factorial(2,f)` should return $5!$
 
+Try to use lambda expressions to define the function `f`.
+
 ```
 
 ```{solution-start} exercise_1