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Q.

What is the cube of 84?


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a

592704

b

692704

c

593704

d

693704 

answer is A.

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Detailed Solution

The given number is 84.
Using shortcut method.
We know that,
a+b 3 = a 3 +3 a 2 b+3a b 2 + b 3
Let a=8 and b=4.
 Substitute a=8 and b=4 in a+b 3 = a 3 +3 a 2 b+3a b 2 + b 3 . 8+4 3 = 8 3 +3 8 2 4 +3 8 4 2 + 4 3
Find b 3 . b 3 =4×4×4 =64 Take the last digit 4 for the final answer and the remaining number to be added in the next step.
Find 3a b 2 . 3a b 2 =3 8 4 2 =384
Add the remaining number in 3a b 2 .
  384+6=390 Take the last digit 0 into the answer next place and the remaining value to be added to the next step.
Find 3 a 2 b. 3 a 2 b=3 8 2 4 =768
Add the remaining number in 3a b 2 . 768+39=807 Take the last digit 7 into the answer next place and the remaining value to be added to the next step.
Find a 3 .
  a 3 =8×8×8 =512
Add the remaining number in a 3 .
  512+80=592
84 3 =592704
Thus the cube of 84 is 592704 and so, option (1) is correct.
 
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