Knowing how to get from one square to the next e.g. knowing that 12x12=144 to 13x13 is useful: 144 + 12 + 13 = 169
I have just learnt this!
I asked my ds1 if he knew it , he Said he just knew his square roots to 20..then proved it!
I then asked him how it worked, and he said without even looking, it just does, who cares, if it works use it!
Told ds2, who is more his mother's son, and he immediately said "how"
I kind of see it of course, but would love a proper explanation please, to relay simply to ds2, as a blathered this morning like mums on the hoof do.
Tinkers, I bet my steel slide rule is older than yours, I have my dad's...it has 12th' s on the top line, with a smaller metric line below! It also has a grease line where he kept it sliding using something yukky and oily.