Question

If the coefficients of $a^{r-1},a^r$ and $a^{r+1}$ in the binomial expansion $(1+a{)}^n$ are in the arithmetic progression, prove that $n^2-n(4r+1)+4r^2-2=0$

Or

The 2nd, 3rd and 4th terms in the expansion of $(x+y{)}^n$ are 240, 720 and 1080, respectively. Find the values of x, y and n.

Answer 1

Or x = 2, y = 3 and n = 5