You need to know the least common denominator (LCD) of 3 and 7 if you want to add or subtract two fractions with 3 and 7 as denominators.
The least common denominator, also called lowest common denominator (LCD), of 3 and 7 is 21.
Here is a math problem example where you need to know the LCD of 3 and 7 to solve:
3/3 + 2/7 = ?
Step 1) Take the LCD and divide each denominator by it as follows:
21/3 = 7
21/7 = 3
Step 2) Multiply each nominator with the respective answers from Step 1:
3 x 7 = 21
2 x 3 = 6
Step 3) Put it all together to solve the problem:
21/21 + 6/21 = 27/21
= 3/3 + 2/7 = 27/21
It's that easy! Once again, the lowest common denominator (LCD) of 3 and 7 is as follows:
21
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