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Using the method of successive subtraction, examine whether or not the following numbers are perfect cubes:
(i) 130
(ii) 345
(iii) 792
(iv) 1331.
To do:
We have to find the cube roots of the given numbers by successive subtraction and examine whether or not the given numbers are perfect cubes.
Solution:
(i) $130 - 1 = 129$
$129 -7 = 122$
$122 -19 = 103$
$103 -37 = 66$
$66 - 61 = 5$
Here, 5 is left.
Therefore,
130 is not a perfect cube.
(ii) $345 - 1 = 344$
$344 - 7 = 337$
$337 - 19 = 318$
$318 - 37 = 281$
$81 - 61 =220$
$220- 91 = 129$
$129 - 127 = 2$
Here, 2 is left.
Therefore,
345 is not a perfect cube.
(iii) $792 - 1 = 791$
$791 - 7 = 784$
$784 - 19 = 765$
$765 - 37 = 728$
$728 - 61 = 667$
$667 - 91 = 576$
$576 - 127 = 449$
$449 - 169 = 280$
$280-217=63$
Here, 63 is left.
Therefore,
792 is not a perfect cube.
(iv) $1331 - 1 = 1330$
$1330 -7 = 1323$
$1323 - 19 = 1304$
$1304 - 37 = 1267$
$1267 - 61 = 1206$
$1206 - 91 = 1115$
$1115 - 127 = 988$
$988 - 169 = 819$
$819 - 217 = 602$
$602 - 271 = 331$
$331 - 331 =0$
Therefore,
1331 is a perfect cube.