Re: Subtracting Fractions

Hello, dsDoan!

Quote:

$\displaystyle \frac{-7x - 49}{x^2 + 2x-35} - \frac{x+7}{5-x}$

We have: .$\displaystyle \frac{-7x-49}{(x-5)(x+7)} - \frac{x+7}{{\color{red}-}(x-5)} \;=\;\frac{-7x-49}{(x-5)(x+7)} \;{\color{red}+}\; \frac{x+7}{x-5} $

. . . . . .$\displaystyle =\;\;\frac{-7x-49}{(x-5)(x+78)} + \frac{x+7}{x-5}\cdot {\color{blue}\frac{x+7}{x+7}} \;\;=\;\;\frac{-7x-49}{(x-5)(x+7)} + \frac{(x+7)^2}{(x-5)(x+7)} $

. . . . . .$\displaystyle =\;\;\frac{-7x-49 + (x+7)^2}{(x-5)(x+7)} \;\;=\;\; \frac{-7x-49 + x^2 + 14x + 49}{(x-5)(x+7)} $

. . . . . .$\displaystyle =\;\;\frac{x^2+7x}{(x-5)(x+7)} \;\;=\;\;\frac{x(x+7)}{(x-5)(x+7)} \;\;=\;\;\frac{x}{x-5}$

Re: Subtracting Fractions

My mistake was with a negative sign from the start, so I'd like to get that issue straightened out:

$\displaystyle -\frac{x+7}{5-x}\;=\; +\frac{x+7}{x-5} $

Going from step one to step two, a negative one has been distributed to the denominator. Why does this make the fraction positive?

1 Attachment(s)

Re: Subtracting Fractions

we can also do it as shown in the attachment.Attachment 27744

Re: Subtracting Fractions

Quote:

Originally Posted by

**ibdutt**

This is the method I originally used when starting this section, but cancelling early in the equation doesn't always lead to two common denominators so I discontinued this approach. If I realize, early on, that cancelling early will result in common denominators it would save time, so I'll keep this in mind.

I would still like to clear up my issue with the negative. It seems like such a trivial issue that will be an ongoing problem if I don't get it sorted now.

Re: Subtracting Fractions

There are two ways to look at it. One is that we multiply and divide by negative one OR alternatively be tale negative one common. In both cases we will have negative into negative and that is positive.

Re: Subtracting Fractions

Quote:

Originally Posted by

**ibdutt** There are two ways to look at it. One is that we multiply and divide by negative one OR alternatively be tale negative one common. In both cases we will have negative into negative and that is positive.

Can you explain this by applying it to the example I posted in post #3?