正g gm z-plane 网-（网份n-可 Two-sided exponential sequence Re 1 ROC,pole-zero-plot Compared with Ex.3.3:Sum of two exponential sequences 2424 ROC, pole-zero-plot   ( )   ( )   1 1 1 3 2 n n x n u n u n = − − − − ( ) ( )( ) 1 1 1 2 12 , 1 1 1 1 3 2 z z z z − − − = + − ( ) 1 1 1 1 1 1 1 1 3 2 X z z z − − = + + − 1 1 3 : 2 ROC   z Two-sided exponential sequence Compared with Ex.3.3: Sum of two exponential sequences