Find a formula for

Question

1/2, -1/4, 1/6, -1/8, 1/10...

So I know that the sequence is just adding 2 to the bottom but i don't know how to write it with a formula including n's..im stuck

Answer 1

This is a geometric sequence where
a=1/2 and r = -1/2

t(n) = ar^(n-1)
= (1/2)(-1/2)^n-1)
= (-1)^(n-1)*(1/2)^n