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

I found an answer from math.stackexchange.com

**18th** derivative of $\arctan(x^**2**)$ at point $x=0$ - Mathematics Stack ...

Hint. One may write d**18**dx**18**arctan(x**2**)=d17dx17**2**x1+x4. then, by a partial fraction decomposition, one has **2**x1+x4=1**2**ℜ(ix−1−i√**2**)−1**2**ℜ(ix−1+i√**2**). Then using.

For more information, see **18th** derivative of $\arctan(x^**2**)$ at point $x=0$ - Mathematics Stack ...

I found an answer from news.yahoo.com

93-year-old Florida mayor prepares run for **20th term** in office

Dec 20, 2013 **...** By Barbara Liston ORLANDO, Florida (Reuters) - A 93-year-old Florida man believed to be the country's oldest mayor is seeking re-election ...

For more information, see 93-year-old Florida mayor prepares run for **20th term** in office