wolfram mathematica - Adding the last two elements of a list -

in mathematica, cleanest way of taking list

{r1, r2, r3, ..., rn, a, b} 

and returning

{r1, r2, r3, ..., rn, + b} 

or more

{r1, r2, r3, ..., rn, f[a, b]} 

for function f?

lst = {a[1], a[2], a[3], a[4], a[5], a[6], a[7], a, b};  lst /. {a___, b_, c_} -> {a, f[b, c]}   ==> {a[1], a[2], a[3], a[4], a[5], a[6], a[7], f[a, b]} 

or (ugly):

append[take[lst, {1, -3}], f @@ lst[[{-2, -1}]]]