当n=4m,cos(nπ/2)=1 n=2m+1,cos(nπ/2)=0 n=4m+2,cos(nπ/2)=-1当n=2m,cos(nπ/2)=0 n=4m+1,cos(nπ/2)=1 n=4m-1,cos(nπ/2)=-1
