Let's solve the given problem step-by-step using the provided values.
1. We are given the expression \( a_n = \ \) and \( a_1 = \ \). However, due to insufficient information, we can't define \( a_n \) and \( a_1 \) with the data at hand. Thus, for now, we will leave \( a_n \) and \( a_1 \) as undefined.
2. We then move to compute \( T_c \). The given formula is:
[tex]\[
T_c = \frac{18 + }{2}
\][/tex]
Here, one of the values in the numerator is missing, but the expression involves finding an average. For the purposes of this solution, we will assume that the second value needed to compute the average is 0, as no specific number is provided.
3. Plugging in the assumed value, we get:
[tex]\[
T_c = \frac{18 + 0}{2} = \frac{18}{2} = 9.0
\][/tex]
From this calculation, we can conclude that:
[tex]\[
a_n = \ \text{None} \\
a_1 = \ \text{None} \\
T_c = 9.0
\][/tex]
So, the final answers based on the given information are:
[tex]\[
a_n = \ \text{Undefined} \\
a_1 = \ \text{Undefined} \\
T_c = 9.0
\][/tex]
