
arithmetic sequence
Meredith starts a saving fund for her son. She deposits $10 on his first birthday, $20 on his second, $30 on his third and so on....for 40 years (wow, is he still going to be living with her, too? Lol *ahem*....).
Use to determine how much she will have deposited in total (ignore interest at this point).
I don't understand how to plug this into the formula, example:
, which equals $320, but logic tells me that $10 + $20 +30, only equals $60.
What am I missing?


Hi Soroban!
That first formula, could it be a typo and it should read n1? That's the closest thing I can find in my book and it is the one on my pretest sheet that they have given me....
BUT the formula you gave is RIGHT! It works on every term I have tried! THANK YOU!

And Soroban, can you help with part 2?
If Meredith's first deposit of $10 earns 5% interest, compounded monthly, what will the final deposit be?
Will it work if I tack on the end of that formula you gave me:
, where n=40?

Actually, n=480, correct? Because it will be 12 x 40?