The coefficient of xm in (1+x)m+(1+x)m+1+.....+(1+x)n, m £ n is
n + 1Cm + 1
n - 1Cm - 1
nCm
nCm + 1
mCm + m+1Cm+ m+2Cm + ..........+ nCm = m+1Cm+1 + m+1Cm+ m+2Cm + ....+ nCm = n+1Cm+1