Squaring two digit numbers in your head

Re:Squaring two digit numbers in your head by ZeroGravitas on Monday March 29, 2010 @05:56AM (#1226199)

Works even better for squares ending in 5 because you'll round up and down to multiples of ten which means the number ends in 00 and you'll always be adding 25.

So:

  35 squared is 4x3 x100 = 12 00 + 25 = 1225

  45 squared is 5x4 x100 = 20 00 + 25 = 2025

  55 squared is 6x5 x100 = 30 00 + 25 = 3025

When I heard about this trick it was referred to as vedic, hindu or indian math, but I don't know if that's actually historically/geographically accurate or not. There's some Japanese nintendo DS games that aim to train you in it, I think it was a bit of a craze over there.

[ Parent | on HN ]

Re:Squaring two digit numbers in your head by tokenadult on Monday March 29, 2010 @07:40AM (#1226300)

When I heard about this trick it was referred to as vedic, hindu or indian math, but I don't know if that's actually historically/geographically accurate or not.

That is not historically accurate, as several authors in India have pointed out. Let me see if I can find a citation:

http://books.google.com/books?id=DHvThPNp9yMC&pg=PA16...

(That's footnote 7 from page 16 of Mathematics in India by Kim Plofker, citing works by scholars in India who have dug into the primary sources.)

[ Parent | on HN ]

Re:Squaring two digit numbers in your head by zellux on Monday March 29, 2010 @09:22AM (#1226450)

The trick applies to two-digit multiplication xy * xz while y + z = 10. And we can simply calculate like this:

  34 * 36 = 3*4 * 100 + 4*6 = 1224
  57 * 53 = 5*6 * 100 + 7*3 = 3021

For multiplications like yx * zx as long as y + z = 10, we need to add x * 100, like this:

  24 * 84 = 2*8 * 100 + 400 + 4*4 = 2016
  46 * 66 = 4*6 * 100 + 600 + 6*6 = 3036

[ Parent | on HN ]

Re:Squaring two digit numbers in your head by hackermom on Monday March 29, 2010 @12:22PM (#1226817)

The two game series are called Brain Age, and Big Brain Academy. Both are great fun - and helpful.

[ Parent | on HN ]

Re:Squaring two digit numbers in your head by jerf on Monday March 29, 2010 @06:00PM (#1227685)

While those are fun, IIRC (and I've spent some time with both) neither of them do large multiplication, and I'm quite confident neither of them try to teach this technique to you; they just present math problems with no technique. (Memorization is appropriate for anything up to 12x12 or 15x15 anyhow, and if they exceed that it's not by much.)

[ Parent | on HN ]

Re:Squaring two digit numbers in your head by ZeroGravitas on Tuesday March 30, 2010 @05:57AM (#1228729)

Yeah, that's a different thing, the games I was thinking of were:

Pa to Tokeru! India Suugaku Drill DS http://www.yesasia.com/global/pa-to-tokeru-india-suugaku-dri...

Indo Shiki Keisan Drill DS http://www.play-asia.com/paOS-13-71-9g-49-en-70-2fox.html

I think the latter might be the same as the Korean title, Indian Math Brain DS.

For those that don't want to import stuff that they probably can't read (though I find you can usually get by) there's also Make 10: a journey of numbers which is a weird maths based adventure game, kind of Brain Age meets WarioWare:

http://www.nintendo.co.uk/NOE/en_GB/games/nds/make_10_a_jour...

[ Parent | on HN ]