Answer the question 1: See whether the number of molecules is greater than 0 or less than 0. If the number of molecules is greater than 0, there exists "the left limit is negative infinity and the right limit is positive infinity", then x=0 is the second kind of infinite discontinuity. If the number of molecules is less than 0, there exists "the left limit is positive infinity and the right limit is negative infinity", so x=0 is still the second kind of infinite discontinuity. In short, x=0 is the discontinuity of the second kind of infinity.

Answer question 2: The limit sinx \ x = 1 cannot be removed, and it can be used directly in operation.