arithmetic series - the expression formed by adding the terms of an arithmetic sequence
Use is the sum of the first n terms.
Write a rule for the nth term using the a1 and d values.
Find the nth term by substituting the value of n into the rule.
Substitute the n, a1, and an values into to find the sum.
Substitute the sum for Sn.
Substitute the first term for a1.
Substitute the rule for an.
Solve for n.
If you get a quadratic equation, solve by factoring or the quadratic formula.
Choose the positive value.