Ex.3.2 Left-sided expo 乎m 3-plane Unit circle x(n]=-a"u[-n-1] X(z) -s-a Gray region:a e ROC:<a ROC If a<l o zeros 比较： pole-zero-plot ×:poles z-plane xn]=a"uln Unit circle X(z)= ROC: Re z-a 2>a 17Ex. 3.2 Left-sided exponential sequence 17 ( ) z X z z a = − ROC z a :     1 n x n a u n = − − − ROC : z a  xn a un n = ( ) z X z z a = − ， : zeros  : poles Gray region: ROC 比较: pole-zero-plot If |a|<1