You need to know the least common denominator (LCD) of 12 and 21 if you want to add or subtract two fractions with 12 and 21 as denominators.
The least common denominator, also called lowest common denominator (LCD), of 12 and 21 is 84.
Here is a math problem example where you need to know the LCD of 12 and 21 to solve:
3/12 + 2/21 = ?
Step 1) Take the LCD and divide each denominator by it as follows:
84/12 = 7
84/21 = 4
Step 2) Multiply each nominator with the respective answers from Step 1:
3 x 7 = 21
2 x 4 = 8
Step 3) Put it all together to solve the problem:
21/84 + 8/84 = 29/84
= 3/12 + 2/21 = 29/84
It's that easy! Once again, the lowest common denominator (LCD) of 12 and 21 is as follows:
84
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