Could anyone explain what is meant by [tex] U_n [/tex]
It are just numbers. That is, [itex]U_1,...,U_n[/itex] are just numbers. You need to figure out which numbers they are.
The wording is probably confusing you.
First, did you do (a) ? do it if you didn't
Then, as micromass told you, Un is just a number, well, in this case, you are dealing with a sequence of numbers, U1, ... Un
You have a generic formula for their sum, which is a function of n, being the result of summing the n elements U1,...Un
so, if you subtract the sum of all the elements, U1,...Un-1, what are you left with ?
Hope this helps so that you continue alone
Another way of looking at (a): if you were told that f(n)= n^2+ 4n, how would you find f(n-1)?
Then (b), what is f(n)- f(n-1)?
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