The sum S n of a finite arithmetic series of n terms is S n = n / 2 (a₁+aⁿ) where a₁ is the first term and a n is the (n)th term.
a. Show that S n =n a₁+ n(n-1) / 2 d by replacing a n with its value in terms of (a₁,n) , and d in the above formula.