# Sequences and series..

• September 27th 2008, 09:43 AM
Mathematix
Sequences and series..
So, how do I solve this to find the value of n ?

Un = (-1)^n (n / n + 4) and I know that Un = 7 / 9 ?

So do you times (-1)^n and ( n / n + 4) ??

Someone help me out here, with explanation please!
• September 27th 2008, 09:55 AM
bkarpuz
Quote:

Originally Posted by Mathematix
So, how do I solve this to find the value of n ?

Un = (-1)^n (n / n + 4) and I know that Un = 7 / 9 ?

So do you times (-1)^n and ( n / n + 4) ??

Someone help me out here, with explanation please!

What is the question exactly?
I did not understand anything about it.
You have $u_{n}=(-1)^{n}\frac{n}{n+4}$ and what do you want to solve?
It seems $u_{n}$ is already in an explicit form.
• September 27th 2008, 11:34 AM
Mathematix
^ From that, I need to know how to find the value of n.
• September 27th 2008, 12:55 PM
bkarpuz
Quote:

Originally Posted by Mathematix
^ From that, I need to know how to find the value of n.

Are you asking for which $n$ value $u_{n}=\frac{7}{9}$ holds?

If so, $n=14$.
To find the solution, first consider $n=2k$ then $n=2k-1$ for $k\in\mathbb{N}$.
In the first case, you will find $k=7$ ( $n=14$) and in the second one $k=-\frac{3}{8}\not\in\mathbb{N}$.
• September 27th 2008, 01:05 PM
Jhevon
you are somewhat vague, Mathematix, but if bkarpuz's last post accurately said what you want, then you must solve $\frac n{n + 4} = \frac 79$ for $n$. and note that $n$ must be even. so if you get an odd number, you are wrong