SOLUTION: In an arithmetic series, t10=21 and t19=48 Find a, d, and S23 Thank You

Question 928076: In an arithmetic series, t10=21 and t19=48
Find a, d, and S23

Thank You
Answer by KMST(5328) (Show Source): You can put this solution on YOUR website!
I assume = first term, = tenth term, = term number , and = sum of first terms.
It is commonly understood that = common difference.
, so
--->--->--->--->
Then, --->--->--->---> .
There are different formulas to calculate sums of arithmetic sequences, but I prefer to remember that adding up the first terms twice, grouping them in pairs "head-to-tail", , , , and so on, you end up with a sum of pairs that all add up to , so
--> .
So, and .
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