# Simplifying Expressions

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• Jul 6th 2008, 04:20 PM
auslmar
Simplifying Expressions
Hello,

I wasn't sure if this belongs in Pre-Calc, Algebra, or elsewhere, so I hope it doesn't get deleted.

I have a couple of expressions I'm trying to simplify and either I'm erring in the simplification, or I am not boiling them down to their "simplest" forms.

I have $a_k = \frac{2(-1)^k2^k}{k!}$ and I need to find $|\frac{a_{k+1}}{a_k}|$

So,

$\frac{\frac{2(1)^{k+1}(2)^{k+1}}{(k+1)!}}{\frac{2( 1)^k(2)^k}{k!}}$ $= \frac{2(1)^{k+1}(2)^{k+1}k!}{2(1)^k(2)^k(k+1)!}$ = $\frac{2^{k+1}k!}{2^k(k+1)!}$ which is not recognized as correct by the system.

There is also another expression $b_k = \frac{2(-1)^kcos(k\pi)}{k}$ for which I need to find $|\frac{b_{k+1}}{b_k}|$

So,

$\frac{\frac{2(1)^{k+1}cos((k+1)\pi)}{k+1}}{\frac{2 (1)^kcos(k\pi)}{k}}$ $= \frac{2(1)^{k+1}cos((k+1)\pi)k}{2(1)^kcos(k\pi)(k+ 1)}$ $= \frac{cos((k+1)\pi)k}{cos(k\pi)(k+1)}$

I'm not sure where I'm faltering. If anyone could point out my mistakes, I would be grateful. Again, sorry if this is in the wrong section.

Thanks for your help,

Austin
• Jul 6th 2008, 05:23 PM
topsquark
Quote:

Originally Posted by auslmar
Hello,

I wasn't sure if this belongs in Pre-Calc, Algebra, or elsewhere, so I hope it doesn't get deleted.

I have a couple of expressions I'm trying to simplify and either I'm erring in the simplification, or I am not boiling them down to their "simplest" forms.

I have $a_k = \frac{2(-1)^k2^k}{k!}$ and I need to find $|\frac{a_{k+1}}{a_k}|$

So,

$\frac{\frac{2(1)^{k+1}(2)^{k+1}}{(k+1)!}}{\frac{2( 1)^k(2)^k}{k!}}$ $= \frac{2(1)^{k+1}(2)^{k+1}k!}{2(1)^k(2)^k(k+1)!}$ = $\frac{2^{k+1}k!}{2^k(k+1)!}$ which is not recognized as correct by the system.

There is also another expression $b_k = \frac{2(-1)^kcos(k\pi)}{k}$ for which I need to find $|\frac{b_{k+1}}{b_k}|$

So,

$\frac{\frac{2(1)^{k+1}cos((k+1)\pi)}{k+1}}{\frac{2 (1)^kcos(k\pi)}{k}}$ $= \frac{2(1)^{k+1}cos((k+1)\pi)k}{2(1)^kcos(k\pi)(k+ 1)}$ $= \frac{cos((k+1)\pi)k}{cos(k\pi)(k+1)}$

I'm not sure where I'm faltering. If anyone could point out my mistakes, I would be grateful. Again, sorry if this is in the wrong section.

Thanks for your help,

Austin

The problem might simply be that you can simplify each of these still more:
$\frac{2^{k+1}k!}{2^k(k+1)!} = \frac{2}{k + 1}$

and
$\frac{cos((k+1)\pi)k}{cos(k\pi)(k+1)} = -\frac{k}{k + 1}$

-Dan