# Adding and Subtracting Rational Expressions

• Apr 7th 2009, 07:48 PM
complicated
Need help. I had this stuff the other day but seem to have forgotten and need review. Here's the problem...

5 over 4xy + 5 over 12yz

I think it's because it's three variables that I don't even know where to start. Most of my other problems like this have been trinomials with 1 or 2 variables.
• Apr 7th 2009, 07:56 PM
Jhevon
Quote:

Originally Posted by complicated
Need help. I had this stuff the other day but seem to have forgotten and need review. Here's the problem...

5 over 4xy + 5 over 12yz

I think it's because it's three variables that I don't even know where to start. Most of my other problems like this have been trinomials with 1 or 2 variables.

in general: $\frac ab \pm \frac cd = \frac {ad \pm bc}{bd}$

here, think of $a = 5,~b = 4xy,~c = 5, \text{ and } d = 12yz$

what can you come up with?

or alternatively, you can begin by finding the LCD: 12xyz

do you know how to continue from there?
• Apr 7th 2009, 08:04 PM
complicated
So it'd end up being 60yz + 20xy over 48xyz?

That's what I got following what you showed me, but I wasn't sure if there was any factoring involved with something like that. The steps my book uses seem a lot more complicated and talks about finding the least common denominator and such.
• Apr 7th 2009, 08:07 PM
Jhevon
Quote:

Originally Posted by complicated
So it'd end up being 60yz + 20xy over 48xyz?

That's what I got following what you showed me, but I wasn't sure if there was any factoring involved with something like that. The steps my book uses seem a lot more complicated and talks about finding the least common denominator and such.

no, it would be $48xy^2z$ for the denominator. now, you can simplify this.

for the other method, you don't have to simplify. but it is not as straight forward. you should know how to find LCDs anyway though
• Apr 8th 2009, 05:12 AM
stapel
Quote:

Originally Posted by complicated
I had this stuff the other day but seem to have forgotten and need review.

For additional review (including worked examples), try some online lessons on adding rational expressions (that is, adding polynomial fractions). (Wink)