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.
b. Explain why S n =n a₁+ n(n-1) / 2} d makes sense by explaining how you can extract each of n a₁ and n(n-1) / 2d from the sum (a₁+ (a₁+d)+ ( a₁+2 d)+ . . . + (a₁²+(n-1) d ).