the sum of first 2 terms of a g. p is -1 and sum of 4 terms is -5. find g. p

let G.P. are a , ar , ar^2 , ... , ar^(n-1)
- 1 = a(r^2 -1)/(r-1) = a(r -1)(r +1)/(r -1) = a(r +1)
- 5 = a(r^4 -1)/(r -1) = a(r^2 -1)(r^2 +1)/(r -1) = a(r -1)(r +1)(r^2 +1)/(r -1) = a(r +1)(r^2 +1)
    = (- 1)(r^2 +1)
r^2 + 1 = 5  =>  r^2 = 4 
=>  r = 2   =>  a = - 1/3  =>  G.P. is   - 1/3  ,  - 2/3  ,  - 4/3  , ..
or   r = - 2 =>  a = 1       =>  G.P. is      1   ,  - 2      ,   4  ,  - 8   ,  ...