In a geometric sequence, each term is multiplied by a ratio, r, to get to the next term. If we know what r is, then we can find any term, including the first term. The challenge is in finding r. If you know any two terms, count how many times the earlier must be multiplied by r in order to arrive at the latter, and set up an equation. From there, solve for r and "back count" or use the explicit formula to find the first term. Look out if you have an even number of r's because you'll have two possible sequences (one where r is positive, and one where r is negative.)
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