let z be the no of carbon-carbon double bonds
type 1: when atoms or grps attached at the end of the carbon chain are different
total no of geometrical isomers = 2^z
type 2: when atoms or grps attached at the end of the carbon chain are same
total no of geometrical isomers = 2^(z-1) + 2^{(z/2)-1} if z is even
=2^(z-1) +2^{(z-1)/2} if z ie odd