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Mental Calculation. In Public School of S. A. Rachinsky - Nikolay Bogdanov-Belsky (1895)
(upload.wikimedia.org)
Wander the gallery. Look at the art. Be polite. If you feel able please post some great art :)
Never seen this before. It’s incredible!
I wish I'd spent more time as a kid doing mental math.
I'm terrible at it, but it is a useful skill despite having a phone in my pocket almost all the time.
I would never have guessed that result! What a weird coincidence.
I'm sure there are lots of little coincidences like this.
Taking x=10, the top of the fraction simplifies to:
5x^2^ + 20x +30
The rest of the solution is left as an exercise for the reader.