# Couple binomial Things

• Jan 24th 2008, 12:39 PM
Gasser666
Couple binomial Things
First, can some tell me if I did this right, and if not, tell me what I did wrong.

expand (x+4y)^5

I got: x^5 + 20x^4y + 26x^3y^2 + 74x^2y^3 + 261xy^4 + 1024y^5

Second, I've forgotten the formulas for exponential growth and decay, If someone could provide those I'd greatly appreciate it.
• Jan 24th 2008, 12:46 PM
Peritus
using the binomial formula:

$\displaystyle \begin{gathered} \left( {x + 4y} \right)^5 = \sum\limits_{k = 0}^5 {\left( {\begin{array}{*{20}c} 5 \\ k \\ \end{array} } \right)} x^{5 - k} \left( {4y} \right)^k \hfill \\ \hfill \\ where:\quad \left( {\begin{array}{*{20}c} 5 \\ k \\ \end{array} } \right) = \frac{{5!}} {{k!\left( {5 - k} \right)!}}\quad \hfill \\ \end{gathered}$
• Jan 24th 2008, 12:49 PM
Gasser666
ummm
I used Pascal's triangle, I was wondering if my numerical coefficients are correct mostly, what you said went over my head, thanks for trying to help me anyways :) and still looking for exponential growth and decay formulas, they have something to do with half-life I think.
• Jan 24th 2008, 02:07 PM
Peritus