设X1（z)和X2（z)为X1（z) =x1[0] +x1[1] z^-1+…+x1[N1]z^-N1X2（z) =x2[0] +x2[1] z卐

题目：设X1（z)和X2（z)为X1（z) =x1[0] +x1[1] z^-1+…+x1[N1]z^-N1X2（z) =x2[0] +x2[1] z卐

设X1(z)和X2(z)为
X1(z) =x1[0] +x1[1] z^-1+…+x1[N1]z^-N1
X2(z) =x2[0] +x2[1] z^-1+…+x2[N2]z^-N2
定义Y(z)=X1(z)X2 (2)
并令-1+…+x1[N1]z^-N1X2（z) =x2[0] +x2[1] z卐" title="设X1（z)和X2（z)为X1（z) =x1[0] +x1[1] z^-1+…+x1[N1]z^-N1X2（z) =x2[0] +x2[1] z卐" />(a)用N和N表示M。
(b)用多项式相乘确定y[0]，y[1]和y[2]
(c)用多项式相乘证明：对于0≤k≤M有
-1+…+x1[N1]z^-N1X2（z) =x2[0] +x2[1] z卐" title="设X1（z)和X2（z)为X1（z) =x1[0] +x1[1] z^-1+…+x1[N1]z^-N1X2（z) =x2[0] +x2[1] z卐" />