SOLUTION: the last three terms of an arithmetic sequence with 18 terms are 67, 72,77 . Find the first term and the sum of the series

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Question 999847: the last three terms of an arithmetic sequence with 18 terms are 67, 72,77 . Find the first term and the sum of the series
Found 2 solutions by josgarithmetic, AnlytcPhil:
Answer by josgarithmetic(39618)   (Show Source): You can put this solution on YOUR website!
when n=18, so substitution of this index will give an equation with only A as the unknown, and is solvable.





Answer by AnlytcPhil(1806)   (Show Source): You can put this solution on YOUR website!
The other tutor forgot the sum.

The hard way is to notice that 77-72 = 5 and 72-67 = 5, and then list
them from right to left like this, subtracting 5 each time 

-8 + -3 + 2 + 7 + 12 + 17 + 22 + 27 + 32 + 37 + 42 + 47 + 52 + 57 + 62 + 67 + 72 + 77

And get -8 for the first term

And then add them up, getting 621.

The easy way is to use the formula



where , , 











and the sum formula:









Edwin
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