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October 18th, 2008, 01:52 PM  #1 
Newbie Joined: Aug 2008 Posts: 16 Thanks: 0  Using LCD to Add and Subtract Rational Expressions
My math book has a guide on how to add and subtract two rational expressions with different denominators. The question is: I know the LCD's are and the book then says to multiply the numerator and denominator of by WHY do I multiply the numerator by instead of if is supposed to be the LCD? so both expressions end up looking like: Thanks! 
October 18th, 2008, 10:58 PM  #2 
Global Moderator Joined: Dec 2006 Posts: 20,484 Thanks: 2041  The LCM of the denominators is To change the denominator of each fraction to the LCM, you have to multiply the numerator and denominator of the first fraction by and of the second fraction by Can you see why now? 
October 19th, 2008, 01:33 PM  #3 
Newbie Joined: Aug 2008 Posts: 16 Thanks: 0  Re: Using LCD to Add and Subtract Rational Expressions
I see. So you don't have to multiply by the LCD to GET the denominators to be the same. And the factors you used to get them to be the same equal also be used in the numerators, correct? Thanks! 
October 19th, 2008, 03:33 PM  #4 
Global Moderator Joined: Dec 2006 Posts: 20,484 Thanks: 2041 
Correct. I hope you can see how the factors are obtained.

October 20th, 2008, 07:00 AM  #5  
Senior Member Joined: Jul 2008 Posts: 895 Thanks: 0  Re: Using LCD to Add and Subtract Rational Expressions Quote:
 

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add, expressions, lcd, rational, subtract 
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