# How do you add Rational Expressions when youre in University Math?

1. Oct 11, 2008

### PhysicsHelp12

Cross multiply ...
and then multiply by the common denominator...

how do you do it

do you do it in your head..by cross multiplying

or the other way

It seems like cross multiplying is less formal ...and I dont know if its 'right' to take the shortcut --since you have to do this so much

If you have more than two like with a partial fractions problem ...im obviously going to just mult/divide by the common denominator

But how do you do it in general ...

2. Oct 11, 2008

### symbolipoint

Which kind of institution makes no difference. You use the same properties and axioms with rational expressions addition no matter whether you are in college, community college, or university. Adding rational expressions? Use lowest common denominator and raise either or both rational expressions to higher terms. This is the same idea whether you have rational fractions, complex fractions, rational expressions(using variable).

3. Oct 11, 2008

### symbolipoint

.... in other words, what you learned in Algebra 1 and Algebra 2 still works.

4. Oct 11, 2008

### epkid08

$$\frac{a}{b} + \frac{c}{d}=\frac{da + bc}{bd}$$

5. Oct 12, 2008

### PhysicsHelp12

No...I no how to do it

im just wondering how you do it persoally

6. Oct 12, 2008

### symbolipoint

The post #4 has it correctly. If the rational expressions are composed of few enough characters, then some of us may do one of the steps in our heads, but doing steps in our heads is not a requirement. One can often use a scientific calculator efficiently after writing the rational expression before or after any simplification step. One does what one is comfortable doing as long as the correct axioms and properties are used.