Let a, b, c, d be any four consecutive coefficients in the binomial expansion (1 + x)n,of then
aa+b+cc+d−2bb+c is equal to
Suppose a=nCr,b=nCr+1,C=nCr+2 and
d=nCr+3 now,
aa+b= nCr nCr+nCr+1= nCr n+1Cr+1=r+1n+1
Similarly, bb+c=(r+1)+n+1 and cc+d=(r+2)+1n+1
Thus, aa+b+cc+d−2bb+c=0