To find the first term in the sequence, we need to determine the common ratio between consecutive terms.
Let's analyze the given sequence: 45, 135, 405, 1215.
We can observe that each term is obtained by multiplying the previous term by 3.
So, if 45 is the 3rd term, we can work backward to find the first term:
45 ÷ 3 = 15 (2nd term)
15 ÷ 3 = 5 (1st term)
Therefore, the first term in the sequence is 5.