# Arthimetic progression help

• Apr 20th 2013, 09:50 AM
Benja303
Arthimetic progression help
7. The 17th term of an AP is 22, and the sum of the first 17 terms is 102. Find
the 1st term and the common difference.

• Apr 20th 2013, 10:07 AM
Plato
Re: Arthimetic progression help
Quote:

Originally Posted by Benja303
7. The 17th term of an AP is 22, and the sum of the first 17 terms is 102. Find
the 1st term and the common difference.

Every term of the AP is $\displaystyle a_n=a_1+(n-1)d$.

So $\displaystyle a_{17}=a_1+16d=22.$ and $\displaystyle \sum\limits_{k = 1}^{17} {{a_k}} = \sum\limits_{k = 1}^{17} {\left( {{a_1} + (k - 1)d} \right)} = 17{a_1} + \frac{{16 \cdot 17}}{2}d = ?$

• Apr 20th 2013, 10:08 AM
MarkFL
Re: Arthimetic progression help
The nth term is:

$\displaystyle a_n=a_1+(n-1)d$

The sum of the first n terms is:

$\displaystyle S_n=\frac{n(a_1+a_n)}{2}$

Use the second equation to find $\displaystyle a_1$, then use the first to find $\displaystyle d$. What do you find?
• Apr 20th 2013, 10:51 AM
Benja303
Re: Arthimetic progression help
Ohhh thanks mark. I was doing it was N/2 X the rest. I see . The 2 divides the entire thing. cool. Nice whip by the way.
• Apr 20th 2013, 11:39 AM
MarkFL
Re: Arthimetic progression help
Quote:

Originally Posted by Benja303
Ohhh thanks mark. I was doing it was N/2 X the rest. I see . The 2 divides the entire thing. cool. Nice whip by the way.

They are actually the same:

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

Did you find that $\displaystyle a_1$ is different than the value you cited in your fist post?