These free printable worksheets give the first three terms of an arithmetic sequence and ask for the sum of the first n. Every sum is a whole number, and the answer key shows where the formula comes from rather than only quoting it.
If they get stuck
The answer key prints on a separate page.
The arithmetic key pairs the ends rather than reciting n(a + l)/2. First plus last is the same as second plus second-last, all the way in — which is why the sum is that pair times half the count. A student who has seen the pairing can rebuild the formula; one who has only seen the formula cannot rebuild the pairing.
Every sheet starts at S₅ and the reason is the question rather than the difficulty: the first three terms are printed, so asking for S₃ is asking a child to add up three numbers already on the page. From five, the terms have to be generated before anything can be added.
S₇ for 5, 1, −3, …