To find the value of \( a_1 \) in the exponential growth equation \( a_n = a_1 \cdot r^{n-1} \), we first need to identify the quantities involved:
- After the first day (which we can denote as day 1), the number of bacteria is 50.
- Since the bacteria quadruple each day, the growth factor \( r \) is 4.
In general, the equation for day \( n \) becomes:
\[ a_n = a_1 \cdot r^{n-1} \]
For day 1 (where \( n = 1 \)), we have:
\[ a_1 = a_1 \cdot 4^{1-1} = a_1 \cdot 4^0 = a_1 \cdot 1 = a_1 \]
On this day, we know \( a_n = 50 \):
\[ 50 = a_1 \cdot 1 \]
Therefore, we can conclude:
\[ a_1 = 50 \]
So, the value that would represent \( a_1 \) is 50.