Hello, juliak!
Show that is arithmetic
and hence find and the sum of the first terms.
The first term is: .
The common difference is: .
The term is: .
So we have: .
The sum of the first n terms is: .
So we have: .
Show that 28, 23, 18, 13....... is arithmetic and hence find un and the sum of sn of the first n terms in simplest form.
I got un=23+5n and sn=n/2(51+5n)
but the answers in the book are:
un=33-5n and sn=n/2(61-5n)
so i dont know where I went wrong