Consider\the\,identity\,(1-t)^n(1+t)^n\equiv%20(1-t^2)^n,\,where\,n\,is\,a\,positive\,integer,\,show\,that\,for\,integral\,values\,of\,r,\,
\sum_{k=0}^{2r}(-1)^k\binom{n}{k}\binom{n}{2r-k}=(-1)^r\binom{n}{r}\,\,\,\,provided\,\,\,\,0\leq%20r\leq%20\frac{1}{2}n
Any help would be appreciated :)