Tuesday, March 3, 2009

Ngày "căn số bậc 2"!

Đọc bản tin này trên Scientific American làm tôi ngạc nhiên là trong giới số học, người ta có ngày "Square root day" (ngày căn số bậc 2)! Tại sao 3/3/09 là ngày này? Đơn giản vì 3 bình phương bằng 3 x 3 = 9.

Phải chờ đến năm 2016 (4 tháng 4) mới có một ngày căn số bậc hai nữa. Trong quá khứ thì 9/9/81, 1/1/01 là ngày căn số bậc hai. Nghe qua có vẻ chuyện "tào lao" nhưng cũng đem lại cho đời một thông tin!


Square root day, 3/3/09, is upon us

Math lovers and numerologists take note: Today, March 3, 2009, is square root day.

The unofficial holiday comes around but nine times a century, when the numbers of the calendar align so that the month and day are each equal to the square root of the year as expressed in two-digit form. (In today's case, 3/3/09, 3 is the square root of 9—in other words, 32 = 3 x 3 = 9.)

Square root days arise in every year whose last two digits are a perfect square, or a number whose square root is a whole number: The last square root day was on February 2, 2004 (2/2/04), and the next occurrence will be on April 4, 2016 (4/4/16). A square root day group on Facebook, boasting more than 2,400 members, suggests celebrating the minor mathematical oddity by eating radishes or other roots—cut into spot-on squares, of course.

Because of the nature of perfect squares, the wait time between square root days increases by two years each time as the century unfolds—five years separated the previous square root day from today's, seven years will pass before the next square root day in 2016, and nine years will elapse before the following one in 2025. But after the final square root of this century, September 9, 2081 (9/9/81), there will be a slightly prolonged layover before the 22nd century starts its own run of square root days on January 1, 2101 (1/1/01).


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