No matter how big a number is.... if it is a perfect cube then you can tell the cube root in 5 seconds......
but you have to remember this chart and cubes from 1 to 9......
cube of 1= 1 ( last digit)
cube of 2= 8 (last digit)
cube of 3= 7 ( 27 but last digit is 7)
cube of 4= 4 ( 64 but last digit is 4 )
cube of 5 = 5 ( 125 last digit is 5)
cube of 6 = 6 ( 256 but last digit is 6)
cube of 7 = 3 ( 343 but last digit is 3)
cube of 8 = 2 ( 512 but last digit is 2)
cube of 9 = 9 ( 729)
as you can i have emphasised on the last digit of every number....
there is nothing to learn in this as you can see that all the have their identity as the cube's last digit except 2, 3, 7 and 8.
if you remember this then volla ......
example. 681472 is a perfect cube..
and its cube root is 88
explanation
1. divide the number in group of three starting from the back..
681 472
2. now as the end part has 2 as last digit therefore cube root's last digit must be 8. ( as from the chart)
3. now 681 is more than 512 ( cube of 8) but less than 729 ( cube of 9). therefore the cube root for this no. must lies in 80s....
4. now we know the first and the last digit of our cube root i.e 88
example--- consider this 778688
dividing in groups of 3... 778 688
as 778 is more than 729( cube of 9) and 688 has last digit as 8 , which is the last digit of cube of 2....
therefore the given number is the perfect cube of 92...
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but you have to remember this chart and cubes from 1 to 9......
cube of 1= 1 ( last digit)
cube of 2= 8 (last digit)
cube of 3= 7 ( 27 but last digit is 7)
cube of 4= 4 ( 64 but last digit is 4 )
cube of 5 = 5 ( 125 last digit is 5)
cube of 6 = 6 ( 256 but last digit is 6)
cube of 7 = 3 ( 343 but last digit is 3)
cube of 8 = 2 ( 512 but last digit is 2)
cube of 9 = 9 ( 729)
as you can i have emphasised on the last digit of every number....
there is nothing to learn in this as you can see that all the have their identity as the cube's last digit except 2, 3, 7 and 8.
if you remember this then volla ......
example. 681472 is a perfect cube..
and its cube root is 88
explanation
1. divide the number in group of three starting from the back..
681 472
2. now as the end part has 2 as last digit therefore cube root's last digit must be 8. ( as from the chart)
3. now 681 is more than 512 ( cube of 8) but less than 729 ( cube of 9). therefore the cube root for this no. must lies in 80s....
4. now we know the first and the last digit of our cube root i.e 88
example--- consider this 778688
dividing in groups of 3... 778 688
as 778 is more than 729( cube of 9) and 688 has last digit as 8 , which is the last digit of cube of 2....
therefore the given number is the perfect cube of 92...
ALL MATHS TRICKS CLICK HERE
INTERVIEW PREPARATION CLICK HERE
the method is abit tricky i am giving u an example but before that u have to learn these mod value too...
ReplyDelete0 is 0; 1 is 1; 2 is 8; 3 is 5; 4 is 9; 5 is 4; 6 is 7; 7 is 2; 8 is 6; 9 is 3; 10 is 10.
eg. 1092727 we know that root will have 1 as frst no. and 3 as last...
now u have to start from last number of the cube and subtract the no. next to it (7-2=5).. after this add the next no. to the result(5+7) ... then again subtract the next no. ... then add next....
in short subraction and add will take place alternate...
it sud be like this ( 7-2+7-2+9-0+1= 20)...
after this you have to add or subtract 11 from this no till it comes under 11...
20-11= 9....
4 is the mod of 9 there take 4 ond forget about 9...
our cube root sud be like this... 1 ? 3...
use it like previous way..
3-?+1=4
?=o
so our cube root is 103
cant we do it as :
ReplyDeleteBreak as 1092 and 727
Last part has units digit 7 hence root has last digit 3
Now the first part lies btw 1000 and 13331 hence taking the lower integer 10 as cube root
Thus cube root becomes 103
yep dats the right way
Deletehow can assume this cube root sud be like this... 1 ? 3...
ReplyDeleteuse it like previous way.. from above example
3-?+1=4
?=o..thanks for advance
How to get reverse of these?? I mean cube root of 56 or 29 or 73?? Is there any shortcut for that??
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteNice!!
ReplyDelete