You need to know the least common denominator (LCD) of 21 and 9 if you want to add or subtract two fractions with 21 and 9 as denominators.
The least common denominator, also called lowest common denominator (LCD), of 21 and 9 is 63.
Here is a math problem example where you need to know the LCD of 21 and 9 to solve:
3/21 + 2/9 = ?
Step 1) Take the LCD and divide each denominator by it as follows:
63/21 = 3
63/9 = 7
Step 2) Multiply each nominator with the respective answers from Step 1:
3 x 3 = 9
2 x 7 = 14
Step 3) Put it all together to solve the problem:
9/63 + 14/63 = 23/63
= 3/21 + 2/9 = 23/63
It's that easy! Once again, the lowest common denominator (LCD) of 21 and 9 is as follows:
63
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