Discover a lot of information on the number 21: properties, mathematical operations, how to write it, symbolism, numerology, representations and many other interesting things!

## Mathematical properties of 21

Is 21 a prime number? No
Is 21 a perfect number? No
Number of divisors 4
List of dividers 1, 3, 7, 21
Sum of divisors 32
Prime factorization 3 x 7
Prime factors 3, 7

## How to write / spell 21 in letters?

In letters, the number 21 is written as: Twenty-one. And in other languages? how does it spell?

21 in other languages
Write 21 in english Twenty-one
Write 21 in french Vingt et un
Write 21 in spanish Veintiuno
Write 21 in portuguese Vinte e um

## Decomposition of the number 21

The number 21 is composed of:

1 iteration of the number 2 : The number 2 (two) represents double, association, cooperation, union, complementarity. It is the symbol of duality.... Find out more about the number 2

1 iteration of the number 1 : The number 1 (one) represents the uniqueness, the unique, a starting point, a beginning.... Find out more about the number 1

Other ways to write 21
In letter Twenty-one
In roman numeral XXI
In binary 10101
In octal 25
In US dollars USD 21.00 (\$)
In euros 21,00 EUR (€)
Some related numbers
Previous number 20
Next number 22
Next prime number 23

## Mathematical operations

Operations and solutions
21*2 = 42 The double of 21 is 42
21*3 = 63 The triple of 21 is 63
21/2 = 10.5 The half of 21 is 10.500000
21/3 = 7 The third of 21 is 7.000000
212 = 441 The square of 21 is 441.000000
213 = 9261 The cube of 21 is 9261.000000
√21 = 4.5825756949558 The square root of 21 is 4.582576
log(21) = 3.0445224377234 The natural (Neperian) logarithm of 21 is 3.044522
log10(21) = 1.3222192947339 The decimal logarithm (base 10) of 21 is 1.322219
sin(21) = 0.83665563853606 The sine of 21 is 0.836656
cos(21) = -0.54772926022427 The cosine of 21 is -0.547729
tan(21) = -1.5274985276366 The tangent of 21 is -1.527499