# Urgend Assistance needed: Abstract Algebra

1. Sep 27, 2006

### Hummingbird25

Hi I'm fairly new at abstract algebra and have therefore got stuck with this assignment.

Hope there is somebody here who can help me complete it, because I have been ill these last couple of weeks.

Its goes something like this

b is a number written in base 10

$$b\;= \;b_010^0 + b_110^1 + b_210^2 + \hdots + b_n10^n$$

where $$0 \leq b_j \leq 10$$

(a) prove that 2 divides b if a only if 2 divides b_0.

My Solution:

Let $$b$$ be a number written in base 10 as:

$$b\;= \;b_010^0 + b_110^1 + b_210^2 + \hdots + b_n10^n$$ where $$0 \leq b_i < 10$$

Prove that: .$$2|b \;\Longleftrightarrow \;2|b_0$$
[/quote]
Given: .$$2|b$$, we have:

. . $$b \;=\;10^nb_n + 10^{n-1}b_{n-1} + 10^{n-2}b_{n-2} + \hdots + 10^2b_2 + 10b_1 + b_o \;=\;2k$$ for some integer $$k.$$

. . $$b_o \;=\;2k - \left(10^nb_n + 10^{n-1}b_{n-1} + 10^{n-2}b_{n-2} + \hdots + 10^2b_2 + 10b_1\right)$$

. . $$b_o \;= \;2k\:-\:\left(2\!\cdot\!5\1\cdot\!10^{n-1}a_n + 2\!\cdot5\1\cdot\!10^{n-1}b_{n-1} + \hdots + 2\!\cdot\!5\!\cdot\!10b_2 + 2\!\cdot\!5b_1\right)$$

. . $$b_o \;= \;2k\:-\:2\left(5\!\cdot\!10^{n-1}b_n + 5\!\cdot\!10^{n-1}b_{n-1} + \hdots + 5\!\cdot\!10b_2 + 5b_1\right)$$

. . $$b_o \;= \;2\left(k - 5\!\cdot\!10^{n-1}b_n - 5\!\cdot\!10^{n-1}b_{n-1} - \hdots - 5\!\cdot\!10b_2 - 5b_1\right)$$

The right side is a multiple of 2, hence the left side is a multiple of 2.

Therefore: .$$2|b_o$$

Given: .$$2|b_o$$, then $$b_o = 2k$$ for some integer $$k.$$

Then: .$$b \;=\;10^nb_n + 10^{n-1}b_{n-1} + 10^{n-2}b_{n-2} + \hdots + 10^2b_2 + 10b_1 + 2k$$

. . . . . $$b\;=\;2\!\cdot\!5\!\cdot\!10^{n-1}b_n + 2\!\cdot\!5\!\cdot\!10^{n-2}b_{n-1} + \hdots + 2\!\cdot\!5\!\cdot\!10b_2 + 2\!\cdot\!5\!\cdot\! b_1 + 2k$$

. . . . . $$b\;=\;2\left(5\!\cdot\!10^{n-1}b_n + 5\!\cdot\!10^{n-2}b_{n-1} + \hdots + 5\!\cdot\!10b_2 + 5\!\cdot\!b_1 + k\right)$$

The right side is a multiple of 2, hence the left side is a multiple of 2.

Therefore: .$$2|b$$

Does this look okay ?

Sincerely Yours
Hummingbird25.

Last edited: Sep 27, 2006
2. Sep 27, 2006

### HallsofIvy

Staff Emeritus
This is abstract algebra? I hope it is an introductory problem!
(by the way, it should be $0\le b_j < 10$.)

Hint: 2 divides 10.

3. Sep 27, 2006

### Hummingbird25

Would You say it enough to show that 2 divides 10 ???

Sincerley

Hummingbird

p.s. I'm going into treatment for a heart condition tomorrow, and have to have these calculations finished by the end of today.

So therefore I hope that somebody in there can assist.

Sincerley

Brenda(Hummingbird25)

Last edited: Sep 27, 2006