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how do we know i=3?­ Where in the question does it indicate that i=3?­ Does the term given a3 directly correlate to the term ai which makes i automatically equal to 3?
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In the sequence \(a_1, \ a_2, \ a_3, \ ..., \ a_n, \ ...\), the terms \(a_i\), \(a_{i+1}\), and \(a_{i+2}\) satisfy the equation \(a_{i+2} = 2a_{i+1} + 3a_i\) for all positive integers i. If \(a_3 = 9\) and \(a_5 = 51\), what is the value of \(a_4\)?

A. 12
B. 30
C. 72
D. 129
E. 171
­

how do we know i=3?­ Where in the question does it indicate that i=3?­ Does the term given a3 directly correlate to the term ai which makes i automatically equal to 3?
­
The formula given, \(a_{i+2} = 2a_{i+1} + 3a_i\), is a general rule for the sequence, meaning it applies to any positive integer \(i\). Therefore, it also holds true when \(i = 3\).­
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