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Ok I know this question maybe too easy.

What is the sum of a finite sequence of terms? $$18, 25, 32, 39, ... ,67$$

The answer is $340$.

I use the formula:

$${ S = \frac{n}{2} \times (a_1 + a_n) }$$

$${ n = \frac{a_n-a_1}{7} }$$ whrere $7$ is the difference between every iteration

I get $297.5$

Am I missing something or is the problem set wrong? Any Hint?

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    $\begingroup$ You're forgetting to count the first element when you calculate n. So it's actually $1+\dfrac{a_n-a_1}{a_2 - a_1}$ $\endgroup$ – Dleep Jul 24 '15 at 1:51
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It is rather $$ a_n=a_1+(n-1)\times 7 $$ giving $$ n \color{red}{-1}= \frac{a_n-a_1}{7} $$ thus here $$n=\frac{67-18}{7}+\color{red}{1}=\color{blue}{8}. $$

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There are 8 terms. So n = 8. That will give you the right answer

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