Answer:
8/3
Explanation:
The sequence could be a geometric sequence with first term 9 and common ratio 2/3. If that's what it is, the next term is 2/3 of 4, or 8/3 = 2 2/3.
The general term of such a sequence is represented by the exponential function ...
an = 9·(2/3)^(n-1)
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You can pick any number you like for the next term and then write a polynomial that will describe the sequence up to and including that number. So, to answer this question sensibly, we have to assume the sorts of sequences you're studying are arithmetic and geometric.
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The attached graph shows both exponential and quadratic formulas for the given numbers.
If the sequence is quadratic, the next two numbers are both 3.