Find the value of nCn−r.
Find the coefficient of xn−2 in (nC0+nC1x+nC2x2.....nCnxn)×(nC0+nC1x+nC2x2.....nCnxn)
here r/nCr=(n-r)/nCn-r how it is possble because its true that nCr=nCn-r but how n-r=r as in numerator