Need some MATH help....simple addition :)

[heat23]

The sum of 2 positive irrational numbers is irrational...

i need a counterexample (2 positive numbers whose sum is rational)

 

[Thorsky]

OK...let x=sqrt(2) and y=2-sqrt(2). Then, x and y are positive and their
sum is 2. x is irrational (take this as known or read a proof in any basic analysis book). y is also irrational. To see this, you can either use the fact that the rationals are closed under addition, or reason as follows:

Suppose y is rational.
Then there is a positive integer m such that my=n for some positive integer n.
So m*(2-sqrt(2))=n, which means (2m-n)/m=sqrt(2), which is impossible since
(2m-n)/m is rational and sqrt(2) isn't.

 

[dopcombo]

nice...

 

[CarpeDeo]

(= math major?

 

[Hanpan]

Probably not. That looks more like discrete math. (most comp sci students take it) but i could be wrong.

 

[CarpeDeo]

Ahh- discrete math (Math 55 at Berkeley). I think I have to take that class soon. bleck.

 

[heat23]

yup this is discrete..
man im really screwed on this assignment...

DOES Anyone use the textbook written by EPP???? If your class has any solutions up or know where i can get some..lmk

 

[pamchenko]

owned

 

[Thorsky]

While this problem showed up in a discrete math course, I wouldn't really classify it as "discrete math." It's closer to the kind of thing one would expect to find in a modern (aka abstract) algebra class. My guess is this is a fairly broad-based class that wants to touch on some basic ideas of groups, rings, fields, etc.

Like so many things, this kind of stuff is hard until it "clicks." I think the best advice is to just think about it until it starts becoming clear. If you're still having trouble, finding a tutor or a classmate who took the course (and got higher than a "D" ) would be your best bet. (Or just buy off the instructor. When I taught Calculus, my rates were surprisingly affordable. )

 

[crab]

yeah what he said