# Help with University Algebra Problems

• Oct 1st 2007, 08:44 PM
Joey123
Help with University Algebra Problems
Hello everyone,
Can anyone help me with the problems below, it's very urgent.

1. R is a ring with property that x^2=x for each x in R.

a) Show that each element of R equals its own negative. (Hint: Consider (x+x)^2)

b) Hence show that R is a commutative ring.

2. Find a factorisation of 9+4sqrt(3) into irreducibles, in Z[sqrt(3)]

• Oct 2nd 2007, 08:04 AM
red_dog
Quote:

Originally Posted by Joey123
Hello everyone,
Can anyone help me with the problems below, it's very urgent.

1. R is a ring with property that x^2=x for each x in R.

a) Show that each element of R equals its own negative. (Hint: Consider (x+x)^2)

b) Hence show that R is a commutative ring.

a) We have $(x+x)^2=x+x$
But, $(x+x)^2=(x+x)(x+x)=x^2+x^2+x^2+x^2=x+x+x+x$.
Then, $x+x+x+x=x+x\Rightarrow x+x=0\Rightarrow x=-x$.

b) We have $(x+y)^2=x+y$.
On the other side, $(x+y)^2=(x+y)(x+y)=x^2+xy+yx+y^2=x+xy+yx+y$.
Then $x+xy+yx+y=x+y\Rightarrow xy+yx=0\Rightarrow xy=-yx=-(-yx)=yx$.
• Oct 2nd 2007, 03:23 PM
Joey123
Thank you so much red_dog for helping me with Q1, I finally understand it now.

Q2. Find a factorisation of 9+4squareroot(3) into irreducibles, in Z[squareroot(3)]

Your input will be appreciated, Thank you.
• Oct 2nd 2007, 05:42 PM
ThePerfectHacker
Quote:

Originally Posted by Joey123
Q2. Find a factorisation of 9+4squareroot(3) into irreducibles, in Z[squareroot(3)]

A very bad I see how to procede is to write,
$9+4\sqrt{3} = (a+b\sqrt{3})(c+d\sqrt{3})$
Thus,
$(ac+3bd)=9$.
Thus,
$(ad+bc)=4$.

And now try to solve for integers. Guessing is the best way it seems. I played around with these for a few minutes but I did not find anything, maybe you do better.