Finding Sum of First 20 Terms

Algebra 2 · Axiom Academy

EXAMPLE Finding S_ 20 for an Arithmetic Series Summing the first 20 terms of with Find the sum of the first 20 terms of the arithmetic sequence using the arithmetic series sum formula. Excellent work — you found the sum of an arithmetic series using the sum formula. Remember: The sum formula: — memorize this. Find the last term first: use a_n=a_1+(n-1)d before the sum formula needs a_n . It's average count: is the average of all n terms, multiplied by how many terms there are. Much faster than adding: no need to write out and add 20 separate numbers one by one. This formula works for any arithmetic sequence — identify a_1 , d , and n , then plug in.

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