1. ## Arithmetic progression

In an Arithmetic progression of n terms, the common differences is d and the last terms is l .how to find first two terms in d , l , n term?.....
Is it use formula S_n=n/2(a+l) or S_n=n/2(2a+(n-1)d)?..i m confusing abt the way to solve...can anybody help me?

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2. ## Re: Arithmetic progression

Term before last term = l-d
2 terms before last term = l-2d
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2nd term from beginning = l-(n-2)d
1st term = l-(n-1)d

3. ## Re: Arithmetic progression

Originally Posted by goldsomecriss
In an Arithmetic progression of n terms, the common differences is d and the last terms is l .how to find first two terms in d , l , n term?.....
Is it use formula S_n=n/2(a+l) or S_n=n/2(2a+(n-1)d)?..i m confusing abt the way to solve...can anybody help me?
This webpage contains a good summery or the formulas. The notation is a bit different.

To your exact question: $S_n$ stands for the sum of every term in the progression.
It is $\large S_n=\dfrac{n(a+\ell)}{2}=\dfrac{n}{2}(a+\ell)=n \dfrac {(a+\ell)}{2}$.
In the formula, $a$ is the first term, $n$ is the number of terms, and $\ell$ is the last term.