The region of convergence of Z - transform of a signal x(n)=2nu(n)−4nu(−n−1)isa<|z|<b. Then the value of a + b is
2nu(n)Z.T⟷zz−2; ROC|z|>2
−4nu(−n−1)Z.T⟷zz−4; ROC|z|<4
∴ROCis2<|z|<4 a+b=2+4=6