Here , conquering cost is O(1)
and partition algo { O(n) } is the cost for dividing the big problem into small sub-problems.
So, T(n) = O(n) + T(k * n ) + T( (1-k)* n ) + O(1)
Divide cost Sub-problems' cost Combine-Cost
Note: f(n) as u described is Divide + Conquer cost , not just conquer.