arithmetic series

**pacman** · September 1st 2009, 07:30 PM

The fifth and and the fiftieth terms of an arithmetic sequence are 3 and 30, respectively. Find the sum of the first 10 terms.

**pomp** · September 1st 2009, 07:36 PM

Originally Posted by pacman

The fifth and and the fiftieth terms of an arithmetic sequence are 3 and 30, respectively. Find the sum of the first 10 terms.

Let $a_1$ be the first term of the sequence and let $d$ be the difference between terms. We know that $a_5 = 3$ and $a_{15} = 30$ .

Therefore we can deduce that since the terms are in arithmetic progression we must have that $a_5 = a_1 + 4d$ and $a_{15} = a_1 + 14d$ .

We know the values of $a_5$ and $a_{15}$ , so we arrive at

$a_1 + 4d = 3$
$a_1 + 14d = 30$

Two equations in two unknowns, which I'm sure you can solve. Once you solve you will have $a_1$ and $d,$ and you should know where to go from there. Good luck.

**pacman** · September 1st 2009, 10:12 PM

Pomp, excuse me, it is fiftieth not fifteenth. Thanks for your reply, i know now how to continue

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