3,-5,9,-11 what is formula

Question

I have numbers of sequence what is formula

Answer 1

 

 x      y

---------

 1     3

 2    -5

 3     7

 4    -9

 5    11

 x   (2x+1)·(-1)^x+1

 

y = (2x +1)·(-1)^x+1