
What we have here is a geometric progression, because the common ratio is -3.
The n^{th} term of a geometric sequence can be found by using the formula a_{n} = ar^{n-1}
where a is the first term. r is the common ratio, which is -3.
Plugging these values in the above formula we get,
So the 20th term or a_{20}=2\times\left(-3\right)^{20-1}=2\times\left(-3\right)^{19}=-2,324,522,934

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18th derivative of $\arctan(x^2)$ at point $x=0$ - Mathematics Stack ...
Hint. One may write d18dx18arctan(x2)=d17dx172x1+x4. then, by a partial fraction decomposition, one has 2x1+x4=12ℜ(ix−1−i√2)−12ℜ(ix−1+i√2). Then using.
For more information, see 18th derivative of $\arctan(x^2)$ at point $x=0$ - Mathematics Stack ...

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