Wednesday, May 25, 2011

Shortcut to find cube root

some times in quantitative aptitude test finding out cube root can turn in to massive headache , normally the question will be about finding square root of integers and six digits. This technique wont work with numbers more than six digits or which is not a perfect cube. This can really save some time for you.

Ok first lets take an example, say , 117649

The answer is 49, lets see how to find it in less than 20 seconds. To solve this you should know cube root of first ten numbers.

N0 Cube
1 1
2 8
3 27
4 64
5 125
6 216
7 343
8 512
9 729
10 1000

ok, now lets start,

Step 1:  in the above table we can see the cube root of first 10 numbers, The question is to find the cube root of 117649

first divide the above number in to two, Start with the last three numbers, it will be your first half and the remaining number will be your other half, see the example below.

117 | 649

Step 2: now , consider the second part (i,e) 117 , see where it lies in the above table, in the above table it lies between 4 and 5, now we have to take the lowest number of  these two (i,e) 4

Step 3: now, consider the first part 649 , the three digit number ends with 9, in the above table we can see that cube root of 9 gives a answer that ends with 9 , therefore 9 is the second answer

combining step 2 and 3 we get 49, which is the final answer

Thats it,


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