SOLUTION: nth element of sequences -1,2,-4,8 is

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Question 1000370: nth element of sequences -1,2,-4,8 is
Answer by Edwin McCravy(20056) About Me  (Show Source):
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-1,2,-4,8
matrix%281%2C2%2C2nd%2Cterm%29%2Fmatrix%281%2C2%2C1st%2Cterm%29=2%2F%28-1%29=-2

matrix%281%2C2%2C3rd%2Cterm%29%2Fmatrix%281%2C2%2C2nd%2Cterm%29=%28-4%29%2F2=-2

matrix%281%2C2%2C4th%2Cterm%29%2Fmatrix%281%2C2%2C3rd%2Cterm%29=8%2F%28-4%29=-2

So there is a common ratio of -2.  This is a geometric 
sequence with r=-2 and a1=-2.

Substitute in the formula:

an = a1rn-1

an = (-1)(-2)n-1  

an = (-1)(-1*2)n-1

an = (-1)(-1)n-12n-1

an = (-1)1(-1)n-12n-1

an = (-1)1+n-12n-1

an = (-1)n2n-1

Edwin