You need to know the least common denominator (LCD) of 8 and 21 if you want to add or subtract two fractions with 8 and 21 as denominators.
The least common denominator, also called lowest common denominator (LCD), of 8 and 21 is 168.
Here is a math problem example where you need to know the LCD of 8 and 21 to solve:
3/8 + 2/21 = ?
Step 1) Take the LCD and divide each denominator by it as follows:
168/8 = 21
168/21 = 8
Step 2) Multiply each nominator with the respective answers from Step 1:
3 x 21 = 63
2 x 8 = 16
Step 3) Put it all together to solve the problem:
63/168 + 16/168 = 79/168
= 3/8 + 2/21 = 79/168
It's that easy! Once again, the lowest common denominator (LCD) of 8 and 21 is as follows:
168
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