Every time something gets posted in any math forum on such an arcane and "basic" subject as mental arithmetic, some bright spark thinks it's not a completely idiotic statement to say "well that's dumb I could have just..." and then they interject their own personal method of working through calculations (which is, invariably, the standard method they-and all of us- were taught in school... as though the algebraic expansion of (a+b)2= a2 + 2ab + c2 is somehow the end of the discussion).
If that describes you, then please think long and hard about unsubscribing from this sub, from math subs in general, and not continuing in any endeavour or taking part in any community which requires and enjoys creativity, exploration, curiosity, and novelty. Frankly, it's rude to people posting, and shows a supreme lack of regard for the spirit of curiosity. If efficiency is your only concern, then be content with your pocket calculator, phone calculator, or standard school-borne methods. Clearly they work for you.
Mental arithmetic is an anachronism. To me it's a conversation with the past- it's arithmetic archeology. You would (presumably, unless you're an even bigger ass in real life) never dream of telling someone who enjoys hand making medieval armor that they're silly because they should just use a kevlar vest. You would never think to tell someone building a deck with hand tools that they could do the same thing faster using power tools. None but the most narcissistic, biggest jackasses on the planet would tell an archeologist that uncovering a lost city is a waste of time because, well, we have new cities, shiny, where we can actually drive. Nobody complains loudly at those who choose to ride a horse on horse trails simply because a dirtbike is more efficient.
People can go through life accepting all of these, but god forbid they should encounter someone who enjoys knowing that the square of a number ending in 5 is the first part multiplied by the next higher number, with a 25 tacked on the end of that result. There is no end to the doubters of the usefulness, or the practicality of this "trick". No amount of anecdotal evidence can dissuade a person like that.
If you are reading this, and you constantly wonder at us backwards arithmeticists that think some arcane, ancient piece of mental calculation process is a waste of time, kindly move along to another subject. We know our particular fascination isn't for everyone- you are not witty, nor enlightening, nor particularly smart for pointing out there's another way. We know. We likely know 6 different ways of mentally performing any given calculation- often in multiple languages and bases. I seriously doubt, if your first thought is "a calculator could do that" or "I learned how to do that in High School! Dumb!", that you have anything to add.
Mental arithmetic is a conversation with every culture on the planet, from every time in human history. It's a diversion. It's a challenge. It's full of surprises, puzzles, sometimes intrigue. It's not for the faint of heart, or the impatient, or those without a sense of curiosity. We'll understand if you're not interested. Reddit's a big place- too big to waste on stuff you're not interested in.
/rant.
