Question

Find S8.
3+-6+12-...

Answer 1

3 6 12 ...i.e every next term is doubling and ofcourse alternating the sign

Answer 2

i see for the alternate signs....(-1)^(n+1)

3(1 + -2 +4 -....)
Sn = (-1)^(n-1) * 3 * ( 2^0 + 2^1 + 2^2 +.....)
( 2^0 + 2^1 + 2^2 +.....) geometric series Sigma a^r
does it ring any bell?