Task 1 the sum Sn of a finite arithmetic series of n terms is Sn=n/2(a1+an) where a1 is the first term and an is the nth term

a. the sum Sn of a finite arithmetic series of n terms is Sn=n/2(a1+an) where a1 is the first term and an is the nth term
b. explain why Sn=na1+n(n-1)/2 d makes sense by explaining how you can extract each of na1 and n(n-1)/2 d from the sum a1+(a1+d)+(a1+2d)+...+(a1+(n-1)d)