Wednesday, February 24, 2010

mental calculation

the looks on the faces of the students in this 1895 painting by n. bogdanov-belsky are wonderful. i love moments when everyone is really focused on trying to figure something out. they are counting in their heads and feeling hopeful and confident that they can solve the puzzle shown on the board if they just think a little more...


try it - no calculating aids allowed!


via dark roasted blend

ps. the answer is discussed in the comments, so have a go at solving it before you peak!

7 comments:

gabemik said...

2

Boris said...

Oh, the result is interesting!

astropixie said...

i agree! i suspected the answer would be simple, but i couldnt guess it before actually working it out.

Hobbes said...

great stuff, you have to wonder why it is so elegant

Boris said...

It's probably because god intended it when he created the world and the length of the year.
:D

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Anonymous said...

It's not too bad with pen and paper, if you recall the sum of squares is just:

\sum_{m=1}^n m^2 = n(n+1)(2n + 1) / 6

14^2 + .. + 10^2 = (14 . 15 . 29 - 9 . 10 . 19) / 6
= 30 (7 . 29 - 3 . 19) / 6
= 5 (203 - 57)
= 5 . 146
= 5 . 73 . 2
= 365 . 2