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## #1 2013-07-14 10:23:51

bronxsystem
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### Multiplying fractions

Hey all i know this is really basic but i have hit a wall

(d / a+b) * (5a + 5b / 2d^2 + 2d)

i can get the answer but im not sure what im actually doing S:

i khow to multiply problem like (1/2) * (1/2) = 1/4 but what confuses me in this problem is the denominator a+b not sure how to multiply the other denominator with it.

My workings

(d / a+b) * (5a + 5b / 2d^2 + 2d)

i just divide by common factors first step
numerator 5a + 5b to 5 ( a+b)
then denominator 2d (d + 1)

then i just divided both numerator and denominator by common factors (a+b) and d
so i got the answer 5 / 2 (d + 1)

how would you guys solve it? sorry if my reasoning bit over the place, any chance you can show me your working so i can aware myself.

thank you.

Last edited by bronxsystem (2013-07-14 10:25:09)

## #2 2013-07-14 10:35:24

bronxsystem
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### Re: Multiplying fractions

had another one that the method works but again not sure what im doing S:
( a + 3 / a - 5) * (2a - 10 / 3a + 9)

= (a + 3 / a - 5) * 2 ( a - 5) / 3 (a + 3)

then just divide numerator and denominators by common factors and i get 2/3

>< whats this called or what rule is it?

Last edited by bronxsystem (2013-07-14 10:42:23)

## #3 2013-07-14 14:00:00

bobbym

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### Re: Multiplying fractions

Hi;

I would call it cancellation.

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

## #4 2013-07-14 22:30:38

bronxsystem
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### Re: Multiplying fractions

sorry those other 2 post had to much fluff a better question maybe is do you think the steps i take are acceptable?

eg

(3ab^2 / 4) * ( 12/ 18a^2)

just simply multiply numerators then denominators and then simplify

(36ab^2) / (72a^2)

then divide by common factor 36a

= b^2 / 2a

thats all it is right?

not sure why this way works i thought your meant to divide both numerator and denominator by same number S:

(3ab^2 / 4) * (12 / 18a^2)

first divide numerators by common factor 3

4ab^2

then divide denominators by common factor 2
2 (8a^2)

then divide both numerators and denominators by 2
(2ab^2 / 4a^2)

then divide by 2a
( b^2 / 2a)

Last edited by bronxsystem (2013-07-14 22:47:35)

## #5 2013-07-15 02:19:09

bobbym

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### Re: Multiplying fractions

The first method is simpler.

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

bronxsystem
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roger thanks

## #7 2013-07-15 02:27:24

bobbym

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### Re: Multiplying fractions

You are welcome.

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.