Binomial Subjective

Prove that \bf{\sum_{r=1}^{m-1} \frac{2r^2-r(m-2)+1}{(m-r) \binom {m}{r}} = m-\frac 1m } .

I'm not able to arrange it in any useful form. Kindly help.

1 Answers

1708
man111 singh ·

\hspace{-16}\mathbf{\sum_{r=1}^{m-1}\; \frac{2r^2-r(m-2)+1}{(m-r).\binom{m}{r}}=\sum_{r=1}^{m-1}\;\frac{r.(r-m)+(r+1)^2}{(m-r).\binom{m}{r}}}$\\\\\\ $\mathbf{\sum_{r=1}^{m-1}\;\frac{-r(m-r)}{(m-r).\binom{m}{r}}+\sum_{r=1}^{m-1}\;\frac{(r+1)^2}{(m-r).\binom{m}{r}}}$\\\\\\ $\mathbf{\sum_{r=1}^{m-1}\;\frac{-r}{\binom{m}{r}}+\underbrace{\bold{\sum_{r=1}^{m-1}\;\frac{(r+1)^2}{(m-r).\binom{m}{r}}}}_{(II)}}$\\\\\\ Now Put $\mathbf{s=m-r}$,in $\mathbf{(II^{nd})}$ Sum, We Get\\\\\\ $\mathbf{\sum_{s=1}^{m-1}\;\frac{(m-s+1)^2}{s.\binom{m}{m-s}}}$\\\\\\ $\mathbf{\sum_{s=1}^{m-1}\;\frac{m^2+s^2-2ms+1+2m-2s}{s.\binom{m}{m-s}}}$\\\\\\ $\mathbf{=\sum_{s=1}^{m-1}\;\frac{s^2}{s.\binom{m}{m-s}}+(m+1).\sum_{s=1}^{m-1}\;\frac{(m+1)-2s}{s.\binom{m}{m-s}}}$\\\\\\ So $\mathbf{=\sum_{r=1}^{m-1}\;\frac{r}{\binom{m}{r}}+(m+1).\sum_{r=1}^{m-1}\;\frac{(m+1)-2r}{r.\binom{m}{r}}}$\\\\\\ Again Calculate\\\\\\

\hspace{-16}\mathbf{(m+1).\sum_{r=1}^{m-1}\;\frac{(m+1)-2r}{r.\binom{m}{r}}=\frac{m^2-1}{m}+\sum_{r=2}^{m-1}\;\frac{(m+1)-2r}{r.\binom{m}{r}}}$\\\\\\ $\mathbf{=\left(m-\frac{1}{m}\right)+\frac{1}{2}.\sum_{r=2}^{m-1}\;\frac{(m+1)-2r}{r.\binom{m}{r}}+\underbrace{\bold{\frac{1}{2}.\sum_{r=2}^{m-1}\;\frac{(m+1)-2r}{r.\binom{m}{r}}}}_{(II)}}$\\\\\\ Now Again Calculate $\mathbf{(II)^{nd}}$ Sum\\\\\\ $\mathbf{\frac{1}{2}.\sum_{r=2}^{m-1}\;\frac{(m+1)-2r}{r.\binom{m}{r}}}$\\\\\\ Put $\mathbf{t=m-r+1}$ , We Get \\\\\\ $\mathbf{\frac{1}{2}.\sum_{t=2}^{m-1}\;\frac{-\left\{(m+1)-2t\right\}}{(m-t+1).\binom{m}{m-t+1}}=-\;\frac{1}{2}.\sum_{r=2}^{m-1}\;\frac{(m+1)-2r}{r.\binom{m}{r}}}$\\\\\\ (Using opening The Summation)\\\\\\ So $\boxed{\boxed{\mathbf{\sum_{r=1}^{m-1}\; \frac{2r^2-r(m-2)+1}{(m-r).\binom{m}{r}}=\left(m-\frac{1}{m}\right)+0+0}}}$

Your Answer

Close [X]