How do I find the LCD?? I really suck at math!! HELP!!

(One of my HW problems)

7. 5/7 - 2/3 = ? How do i use LCD on them?

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- Sep 17th 2008, 07:05 PMClulessOnMathLCD & Adding/Subtracting Whole Numbers?
How do I find the LCD?? I really suck at math!! HELP!!

(One of my HW problems)

7. 5/7 - 2/3 = ? How do i use LCD on them? - Sep 18th 2008, 03:30 AMParveen KaurLCD in fractions
LCD is least common divisor

7.5/7 - 2/3

Here the denominators are 7 and 3 the lowest common divisor of these is 7 x 3 = 21

Make both denominators 21

7.5/7 x 3/3 - 2/3 x 7/7

= 22.5/21 - 14/21

= 8.5/21 - Sep 18th 2008, 07:18 AMmasters
Somehow, I think the 7 is the problem number. Am I right?

In any case, $\displaystyle \frac{5}{7}-\frac{2}{3}$ has denominators of**7**and**3**.

This one is easy since they are both prime numbers. You just multiply them together to get**21**as your new denominator. Now convert each fraction in your subtraction expression to the new denominator.

$\displaystyle \frac{5}{7}=\frac{?}{21}$ and $\displaystyle \frac{2}{3}=\frac{?}{21}$

Determine what number you need to multiply the denominator by to get the new denominator. In the first one, it would be**3**.

$\displaystyle \frac{5}{7} \cdot \frac{3}{3}=\frac{15}{21}$

In the second one, it would be**7**.

$\displaystyle \frac{2}{3} \cdot \frac{7}{7}=\frac{14}{21}$

Now use your two converted fractions to obtain the difference.

$\displaystyle \frac{15}{21} - \frac{14}{21}=\frac{15-14}{21}=\boxed{\frac{1}{21}}$ - Sep 22nd 2008, 01:07 PMSouljaMaths
LCD of $\displaystyle 5/7 - 2/3$

First you wanne get the same denominator...

You will do this by timing the first fraction by 3 and the second one by 7.

$\displaystyle 15/21 - 14/21$

Then you will just simply Subtract the Fractions like this...

$\displaystyle 15-14/21$

You will then get your answer..

$\displaystyle 1/21$