Can you double-check the question to verify whether it should be something like

?

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

P(k)is

P(k+1)is

Try to express P(k+1) in terms of P(k) to show that P(k+1) must be true if P(k) is