In the exponential growth formula \( a_n = a_1 \cdot r^{n-1} \), where:
- \( a_n \) is the amount at day \( n \),
- \( a_1 \) is the initial amount (amount at day 1),
- \( r \) is the growth factor, and
- \( n \) is the number of days,
we know the following:
- After the first day (\( n = 1 \)), there were 50 bacteria, so \( a_2 = 50 \) after day 1.
- The bacteria quadruple each day, so \( r = 4 \).
To find \( a_1 \) (the initial amount of bacteria at \( n = 1 \)), we can set up the equation for \( n = 1 \):
\[ a_1 = a_1 \cdot 4^{1-1} \]
This simplifies to:
\[ a_1 = a_1 \cdot 4^0 \]
Since \( 4^0 = 1 \), it simplifies to:
\[ a_1 = a_1 \]
This doesn't directly help find \( a_1 \). Instead, recognize that if there are 50 bacteria after the first day, we can conclude that \( a_1 \) must be 50 divided by 4 because it quadruples each day. Hence, we can determine \( a_1 \):
\[ a_2 = 50 = a_1 \cdot 4 \] To reverse that, we have:
\[ a_1 = \frac{50}{4} = 12.5 \]
Thus, the initial amount \( a_1 \) is:
\[ \text{The value of } a_1 \text{ is } 12.5. \]