Cubic Roots

There's a simple way to find the cubed root of a number.

First, look at the last digit of the number. If the number ends in...

0, the last digit ends in 0.
1, the last digit ends in 1.
2, the last digit ends in 8.
3, the last digit ends in 7.
4, the last digit ends in 4.
5, the last digit ends in 5.
6, the last digit ends in 6.
7, the last digit ends in 3.
8, the last digit ends in 2.
9, the last digit ends in 9.

Once you determine the last digit, mentally chop off the last three digits of the number under the cube root sign. Then, use your knowledge of cubic numbers to find out what cubic numbers the remaining digits are between. Write the smaller of these two numbers.

Ex: The cubed root of 373248  = __________.

1) The last digit is 8, so the new number will end in 2.
2) "Chopping off" the last three digits of the number leaves us with 373. What two cubed numbers does this fall between?
3)  7 cubed is 343 and 8 cubed is 512, so the first digit is 7. Write down the 7.
4) Therefore, the answer is 72.

Ex 2: The cubed root of 57392
1) The last digit is 2, so the new number will end in 8.
2) "Chopping off" the last three digits of the number leaves us with 57.
3) 3 cubed is 27, 4 squared is 64. Therefore, the first number is 3.
4) Thus, the number is 38. (*Note: The true cubic root of 57392 is 38.573. This approximates a close answer that will suffice on the Number Sense test for certain problems.)

-Marisa