Source: posting by Ray Langley to MentalCalculation@yahoogroups.com (Wed, Jan 13, 2010 at 8:27 PM); subject: “[Mental Calculation] Cube Root Extraction Trick”
… Hand a calculator to a friend. Ask him to enter a two-digit number. Have him multiply his chosen number by his number, then multiply it again. Let’s say he chooses number 57.
57 X 57 X 57 = 185, 193
To extract the cube root of this number we must first memorize
1 = 1 2 = 8 3 = 27 4 = 64 5 = 125 6 = 216 7 = 343 8 = 512 9 = 729
Notice that :
To derive the cube root of our number (185, 193), we divide it into two sections at the “comma”.
So, the answer is 57.
This feat looks like it requires a great deal of mental ability, but it can be easily learned by anyone in a short time.
- cube root calculation – http://xw2k.nist.gov/dads/HTML/cubeRoot.html in NIST’s Dictionary of Algorithms and Data Structures (http://xw2k.nist.gov/dads/)
- Cube Root Calculation System – http://www.mindmagician.org/cubert.aspx at Mind Magician site (http://www.mindmagician.org/)
- How to Do Cube Roots of 9 Digit Numbers in Your Head (1729.com blog) – http://www.1729.com/blog/CubeRoots.html
- How to Do Cube Roots of 9-Digit Numbers – http://www.ehow.com/how_2049759_do-cube-roots-9digit-numbers.html
- Mental calculation (WikiPedia) – http://en.wikipedia.org/wiki/Mental_calculation
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