# Thread: Having sign problem with algebraic fractions

1. ## Having sign problem with algebraic fractions

Hi,

I'm having a problem with an algebraic fractions equation. It goes:

5/3(v-1) + (3v-1)/(1-v)(1+v) + 1/2(v+1)

The first thing I do is factor out the negative in the first fraction, getting:

-5/3(1-v)

Giving a cd of 6(1-v)(v+1). Now that that is done I multiply the numerators by the necessary factors:

-5*2(v+1)

6(3v-1)

3(1-v)

Which gives me -10v -10 + 18v -6 -3v + 3

Which adds up to 5v - 13

BUT the answer is -5v + 13, and if I factor out the negative in the second equation, all the signs are reversed and the equation works out to the right answer. So I'm confused as to why things didn't work when I factored out the negative in the first fraction.

Thanks,

Kevin

2. ## Re: Having sign problem with algebraic fractions

Hey KevinShaughnessy.

Can you clarify whether (3v-1)/(1-v)(1+v) is (3v-1)(1+v) / (1-v) or (3v-1) / [(1+v)(1-v)]?

3. ## Re: Having sign problem with algebraic fractions

Hello, Kevin!

They approached the problem differently . . . that's all.

$\displaystyle \text{Simplify: }\:\frac{5}{3(v-1)} + \frac{3v-1}{(1-v)(1+v)} + \frac{1}{2(v+1)}$

They factored a "minus" out of the second fraction.

. . $\displaystyle \frac{5}{3(v-1)} - \frac{3v-1}{(v-1)(v+1)} + \frac{1}{2(v+1)}$

The LCD is $\displaystyle 6(v-1)(v+1)\!:$

. . $\displaystyle \frac{5}{3(v-1)}\cdot {\color{blue}\frac{2(v+1)}{2(v+1)}} - \frac{3v-1}{(v-1)(v+1)}\cdot {\color{blue}\frac{6}{6}} + \frac{1}{2(v+1)}\cdot {\color{blue}\frac{3(v-1)}{3(v-1)}}$

. . $\displaystyle =\;\frac{10(v+1)}{6(v-1)(v+1)} - \frac{6(3v-1)}{6(v-1)(v+1)} + \frac{3(v-1)}{6(v-1)(v+1)}$

. . $\displaystyle =\;\frac{10v + 10 - 18v + 6 + 3v - 3}{6(v-1)(v+1)}$

. . $\displaystyle =\;\frac{13-5v}{6(v-1)(v+1)}$

4. ## Re: Having sign problem with algebraic fractions

Originally Posted by chiro
Hey KevinShaughnessy.

Can you clarify whether (3v-1)/(1-v)(1+v) is (3v-1)(1+v) / (1-v) or (3v-1) / [(1+v)(1-v)]?
It's (3v-1) / [(1+v)(1-v)].

5. ## Re: Having sign problem with algebraic fractions

Originally Posted by Soroban
Hello, Kevin!

They approached the problem differently . . . that's all.

They factored a "minus" out of the second fraction.

. . $\displaystyle \frac{5}{3(v-1)} - \frac{3v-1}{(v-1)(v+1)} + \frac{1}{2(v+1)}$

The LCD is $\displaystyle 6(v-1)(v+1)\!:$

. . $\displaystyle \frac{5}{3(v-1)}\cdot {\color{blue}\frac{2(v+1)}{2(v+1)}} - \frac{3v-1}{(v-1)(v+1)}\cdot {\color{blue}\frac{6}{6}} + \frac{1}{2(v+1)}\cdot {\color{blue}\frac{3(v-1)}{3(v-1)}}$

. . $\displaystyle =\;\frac{10(v+1)}{6(v-1)(v+1)} - \frac{6(3v-1)}{6(v-1)(v+1)} + \frac{3(v-1)}{6(v-1)(v+1)}$

. . $\displaystyle =\;\frac{10v + 10 - 18v + 6 + 3v - 3}{6(v-1)(v+1)}$

. . $\displaystyle =\;\frac{13-5v}{6(v-1)(v+1)}$
But aren't the two answers fundamentally different being that one produces a negative number and the other produces the same number but positive?

6. ## Re: Having sign problem with algebraic fractions

Originally Posted by KevinShaughnessy
But aren't the two answers fundamentally different being that one produces a negative number and the other produces the same number but positive?
your answer was$\displaystyle \frac{5v-13}{6{\color{magenta}(1-v)}(1+v)}$ while most probably the answer in your book is given as $\displaystyle \frac{13-5v}{6{\color{magenta}(v-1)}(1+v)}$, Which you know are the same answers because the answer in your book changed your $\displaystyle (1-v)$ to $\displaystyle (v-1$) by multiplying the numerator and denominator by $\displaystyle -1$.
your answer was$\displaystyle \frac{5v-13}{6{\color{magenta}(1-v)}(1+v)}$ while most probably the answer in your book is given as $\displaystyle \frac{13-5v}{6{\color{magenta}(v-1)}(1+v)}$, Which you know are the same answers because the answer in your book changed your $\displaystyle (1-v)$ to $\displaystyle (v-1$) by multiplying the numerator and denominator by $\displaystyle -1$.