Show that the following integers are cubes of negative integers.
Show that the following integers are cubes of negative integers. Also, find the integer whose cube is the given integer.
(i) −5832
(ii) −2744000
In order to check if a negative number is a perfect cube, first check if the corresponding positive integer is a perfect cube. Also, for any positive integer $m,-m^{3}$ is the cube of $-m$.
(i)
On factorising 5832 into prime factors, we get:
$5832=2 \times 2 \times 2 \times 3 \times 3 \times 3 \times 3 \times 3 \times 3$
On grouping the factors in triples of equal factors, we get:
$5832=\{2 \times 2 \times 2\} \times\{3 \times 3 \times 3\} \times\{3 \times 3 \times 3\}$
It is evident that the prime factors of 5832 can be grouped into triples of equal factors and no factor is left over. Therefore, 5832 is a perfect cube. This implies that $-5832$ is also a perfect cube.
Now, collect one factor from each triplet and multiply, we get:
$2 \times 3 \times 3=18$
This implies that 5832 is a cube of 18.
Thus, $-5832$ is the cube of $-18$.
(ii)
On factorising 2744000 into prime factors, we get:
$2744000=2 \times 2 \times 2 \times 2 \times 2 \times 2 \times 5 \times 5 \times 5 \times 7 \times 7 \times 7$
On grouping the factors in triples of equal factors, we get:
$2744000=\{2 \times 2 \times 2\} \times\{2 \times 2 \times 2\} \times\{5 \times 5 \times 5\} \times\{7 \times 7 \times 7\}$
It is evident that the prime factors of 2744000 can be grouped into triples of equal factors and no factor is left over. Therefore, 2744000 is a perfect cube. This implies that $-2744000$ is also a perfect cube.
Now, collect one factor from each triplet and multiply, we get:
$2 \times 2 \times 5 \times 7=140$
This implies that 2744000 is a cube of 140.
Thus, $-2744000$ is the cube of $-140$.