We know that the <span>formula of the arithmetic series is </span>Sn=<span>n[2(a1) + (n - 1)d]/2 where Sn=80 n=20 d=2 a1=? then 80=20*[2*a1+19*2]/2----------------> </span>160=[40*a1+760] 160=[40*a1+760]----------> a1=(160-760)/40------------> a1=-15 <span> the answer is a1=-15
To solve this, we are going to use the standard formula for arithmetic series where: is the sum of the arithmetic sequence is the number of terms is the difference is the first term From our problem we know that , , and . Lets replace those values in our formula to find :
We can conclude that the first term of the arithmetic series is