# Homework Help: Rational expressions

1. Jul 23, 2008

### ramstin

1. x/20 = (3/8)-(4/5)

2. solve

3. My attempt as far as I can tell there is no LCM so

3/8 becomes 15/40 4/5 becomes 32/40

(15/40)-(32/40)= 17/40 which equals 8.5/20 which means x=8.5

For some reason I don't think I got the right answer???

2. Jul 23, 2008

### rock.freak667

$$\frac{15}{40}- \frac{32}{40} \neq \frac{17}{40}$$

15/40 is less than 32/40. Do you think you answer would be positive?

If you have to find 3-4, you won't get 1. You'd get -1.

3. Jul 23, 2008

### ramstin

o.k. how do you get a LCM to factor with if the only LCM is 1?

Man I am confused :P

4. Jul 23, 2008

### rock.freak667

Let's start over then.

$$\frac{3}{8}- \frac{4}{5}$$

We agree that the common denominator is 40,right?

$$\frac{3}{8}- \frac{4}{5}= \frac{15}{40} - \frac{32}{40}$$

Understand up to here right?

Now, if the denominators are the same, you can write it as one single fraction like so:

$$\frac{15}{40} - \frac{32}{40}= \frac{15-32}{40}$$

Now you must find the value of 15-32

5. Jul 23, 2008

### ramstin

so that would be -17

-17/40 so it would be -8.5

6. Jul 23, 2008

### ramstin

I guess what is confusing me is that the assignment was about solving equations after finding and factoring the LCM.

7. Jul 23, 2008

### rock.freak667

Yep..

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