DIR Return Create A Forum - Home
       ---------------------------------------------------------
       gworld
  HTML https://gworld.createaforum.com
       ---------------------------------------------------------
       *****************************************************
   DIR Return to: MATH FACTS
       *****************************************************
       #Post#: 108--------------------------------------------------
       SQUARE ENDING IN5
       By: eba95 Date: July 30, 2010, 6:35 am
       ---------------------------------------------------------
       Squares Ending in 5
       Give me any 2 digit number that
       ends in 5, and I'll square it in my
       head!
       452 = 2025
       852 = 7225, etc.
       There's a quick way to do this: if the
       first digit is N and the second digit is
       5, then the last 2 digits of the answer
       will be 25, and the preceding digits
       will be N*(N+1).
       Presentation Suggestions:
       After telling the trick, have students
       see how fast they can square such
       numbers in their head, but doing
       several examples.
       The Math Behind the Fact:
       You may wish to assign the proof as
       a fun homework exercise: multiply
       (10N+5)(10N+5) and interpret! The
       trick works for larger numbers, too,
       although it may be harder to do this
       in your head. For instance 2052 =
       42025, since 20*21=420. Also, you
       can combine this trick with other
       lightning arithmetic tricks. So 1152 =
       13225, because 11*12 = 132, using
       the Multiplication by 11 trick.
       The reference also contains more
       secrets of fast mental calculations.
       *****************************************************