If this is the case, then the minimum value n can be within the brackets is 2,
since the value 3 tells you that the numbers being summed in brackets are
all increasing by 1.
So your base case ought to be n=2.
To prove this using induction
Try to express P(k+1) in terms of P(k) to show that P(k+1) must be true if P(k) is