Can someone help me with number 15

Step-by-step explanation:

We will evaluate that with n values starting at 1 and ending at 5.  To do that, simply replace the exponent n with 1, then 2, then 3, then 4, then 5.

 so

Then f(2):

simplifies a bit so maybe it's easier to see how to handle this:

so

Then f(3):

And f(4):

And finally, f(5):