List of top Mathematics Questions on Binomial theorem asked in BITSAT

The middle term in the expansion of \( \left(\frac{10}{x} + \frac{x}{10}\right)^{10} \) is:
If $\displaystyle\sum^n_{R = 0} (-1)^r \; \frac{^{n}C_{r}}{^{r+3}C_{r}} = \frac{3}{a+3} $ , then a - n is equal to