Factor each number to see if they have common factors:
21 = 3*7
36 = 3 *12
The LCD is, 36*7 or 21*12, 252.
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If you mean the lowest, it's 2
LCD(9, 1, 4) = 36
21/36 in lowest term is 7/12. Multiply both, the numerator and the denominator, by the same number in order to obtain equivalent fractions.
The lowest common denominator of any two or more whole numbers, such as 15 and 36, will always be one (1) because 'common denominator' refers to the denominators of two or more fractions or mixed numbers, not whole numbers. So, a set of whole numbers (x,x) would have to be converted to their fraction equivalents, i.e., x/1, which will always yield a denominator of 1.
To simplify the fraction 21/36 you would divide the numerator and denominator by the GCF (greatest common factor) of 21 and 36 which is 3. So this simplifies to: 21/36 = (3 * 7)/(3 * 12) = 7/12.