Can you explain to me how to find the "least common denominator"? I don't want the answer! (read more)

Question

Can you explain to me how to find the least common denominator? the fractions are 2/7 and 4/5! Please help me!

Answer 1

You have to look at the common factors.

The factors of 7 are 1 and 7; the factors of 5 are 1 and 5. So the lowest number that has all those factors in common is 35, and the LCD is 35ths.

Here's a better example - how do you find the LCD of 1/8, 1/9 and 1/12? You COULD multiply them together and get 864ths. However, look at the factors:
8 = 2*2*2
9 = 3*3
12 = 3*2*2

So if we pick 2*3*2*3*2 we should cover all three numbers, which equals 72. This is correct - 8, 9, and 12 all go into 72 evenly. [You need 3 2s to cover 8, and two 3s to cover 9]

Hope this helps!

Answer 2

When you work with fractions, you have to convert them into the same thing, otherwise you're working with apples and oranges. So -- given your examples of 2/7 and 4/5 you need to find something that both 7 and 5 can go into evenly (with nothing left over). One way to find that is to multiply the two together, which gives you 35. That works well when you only have small numbers, but if you have larger numbers you don't want to have to convert your fractions into something with a six-digit denominator, so you find the prime numbers that make up each of the denominators. If, for example, the denominator was 35, your prime numbers would be 5 and 7.
You do that for each of the denominators. Then strike out any prime numbers that the two denominators have in common. For example, if one denominator was 35 and the other was 70, your prime numbers for the first one would the 5 and 7. For the second they would be 7,5,2. If you cancel out the 5 and 7 that are shared by both sets of denominator factors, you are left with two. So all you would have to do to even out these denominators is divide 70 by 2, or multiply 35 by 2. Either way, you end up with a common denominator that is much less than you would have had you multiplied 35 by 70.
Have I jumbled this up too much,or are you with me?
Good luck on it. If you can master this basic step, fractions become sooooo much simpler to work with!

Answer 3

Here are the steps I use....

1.) Look at your denominators. The denominator is the bottom number of a fraction. So in this case, it is 5 and 7.

2.) Basically you are looking for the Least Common Multiple of these numbers. (LCM) Start listing the multiples of these numbers side by side.

EX.
5 7
10 14
15 21
20 28
25 35
30 42
35 49
40 56

3.) The least common denominator is the SMALLEST number that is in both lists above. Basically you just list the multiples of the numbers until you find one they have in common.


Why do you need to learn this stuff? :) Because it is essential if you want to add or subtract those two fractions. But that is another answer entirely. Hope this helps, good luck!

Answer 4

Your denominators are 5 and 7. To find the LCD, list the multiples of each until you find the least one they have in common. In this case, 35 is the least common multiple, and therefore the LCD.

5 - 5, 10, 15, 20, 25, 30, 35, 40, 45,...
7 - 7, 14, 21, 28, 35, 42, 49, 56,...

Answer 5

Denominator means the bottom nuber so you just find a number that both 5 and 7 can go into. Or you can mutiply 7 and 5 together and that could be the least common denominator but rember to simplify! GOOD LUCK!