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# Arithmetic series

Walk through a guided practice where you'll start by finding a simple sum and end by evaluating finite arithmetic series.

### Find the sum of $1 + 3 + 5 + 7 + 9$1, plus, 3, plus, 5, plus, 7, plus, 9.

Awesome! You just found the sum of a small arithmetic series. It only had 5 terms. But, what if it had one million terms? We'd want a formula for sure. Thankfully, we've already learned of such a formula.
Identify the formula for the sum of an arithmetic series.

Sweet! So you remember the formula. Now let's make sure we remember how to apply it.
Choose the answer that shows the formula correctly used to find the sum you found.

Alright, so we're feeling good so far. Let's try to use the formula to find the sum of an arithmetic series that would be tedious to calculate by hand.

## Consider the series $3 + 5 + 7 + ... + 401$3, plus, 5, plus, 7, plus, point, point, point, plus, 401.

Find the values of a, start subscript, 1, end subscript and a, start subscript, n, end subscript for this series.
a, start subscript, 1, end subscript, equals
a, start subscript, n, end subscript, equals

Find the value of n for this series.
n, equals

Find the sum of 3, plus, 5, plus, 7, plus, point, point, point, plus, 401

Wow! Okay, looks like you've got this.

## Try it yourself

Problem 1
Find the sum.
11, plus, 20, plus, 29, plus, point, point, point, plus, 4052, equals

Nice! Try another one!
Problem 2
Find the sum.
10, plus, left parenthesis, minus, 1, right parenthesis, plus, left parenthesis, minus, 12, right parenthesis, plus, point, point, point, plus, left parenthesis, minus, 10, comma, 979, right parenthesis, equals