An $n-$year decreasing annuity is payable annually in arrear where the payment at the end of the first

year is $n,$ the payment at the end of the second year is $(n)$ and so on until the final payment at

the end of year $n$ is $1.$

$($a$)$ Show that the present value of this annuity is

$\frac{n-a_{/bar(n)|}}{i}$