计算N的值，怎么总是条件冲突？

sunyuanxin

看图吧，我是这么做的
x[n]=u[n]-u[n-10]
h[n]=u[n]-u[n-N]
y[n]=x[n]*h[n]=(u[n]-u[n-10])*(u[n]-u[n-M])=u[n]*u[n]-u[n]*u[n-M]-u[n-10]*u[n]+u[n-10]*u[n]+u[n-10]*u[n-M]
for u[n-M]*u[n-N]=(n-N-M+1)u[-n+M+N]
y[4]=(5-N)u[-4+n]+(-5)u[6]+(-5-N)u[6+N]=(5-N)u[-4+N]-5-(5+N)u[6+N]=5
if N>-6 y[4]=-5-5-N=5
N=-15 conflict the previous condition. if N>4 y[4]=5-2N=5
N=0 also conflict the previous condition.