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September 24th, 2010, 10:26 PM   #1
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please help adding fractions???

What are the steps?

I've been on this for 7 days and don't know what to do.

Like if there's

1/7+4/8=
lcd is between 41 or 40 for this problem?

Like I know you're supposed to find the lcd for the bottoms to get a common denominator, but
when you find the lcd what's next
???
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September 24th, 2010, 10:54 PM   #2
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Re: please help adding fractions???

First, we should reduce both fractions if possible to simplify the process, so we work with the smallest numbers we can.

1/7 is fully reduced, but 4/8 may be written as 4/(2·4) = 1/2

So we have 1/7 + 1/2. The two denominators share no common factors (both are in fact prime) so the lcd will be 2·7 = 14.

Begin with 1/7 and ask yourself what the denominator 7 must be multiplied by to get 14, and as we saw, that is 2. So we need to multiply 1/7 by 1 (so we don't change its value, only the way its represented) in the form of 2/2.

(1/7)(2/2) = 2/14

Now, consider 1/2. What must the denominator 2 be multiplied by to get 14, which is 7. So as before we multiply by 7/7.

(1/2)(7/7) = 7/14

Now, our two fractions have a common denominator, and we can directly add them together.

2/14 + 7/14 = 9/14

Notice how the numerator is the sum of the original denominators and the denominator is the product of the original denominators?
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September 25th, 2010, 05:28 PM   #3
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Quote:
Originally Posted by alternap18
. . . you're supposed to find the lcd for the bottoms to get a common denominator . . .
You need to use a common denominator, but it needn't be the LCM (least common multiple) of the original denominators. Any other common multiple of the denominators could be used instead.

For example, 5/6 - 7/10 = (5/6)(5/5) - (7/10)(3/3) = 25/30 - 21/30 = 4/30 = 2/15,
or . . . 5/6 - 7/10 = (5/6)(10/10) - (7/10)(6/6) = 50/60 - 42/60 = 8/60 = 2/15.
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