Here’s a cute little poem:
I wish I knew
The root of two.
O charmed was he
To know root three.
So we now strive
To find root five.
The beginning of each stanza is a mnemonic for the number mentioned in the following line.
√ 2 = 1.414
√ 3 = 1.732
√ 5 = 2.236
I found this in Twenty Years Before the Blackboard by Michael Stueben. Steuben sites the sources as Dictionary of Mnemonics, London 1972. He doesn’t give any other information such as author or editor.
Update: Additional verse from the comments.
It rhymes with heaven
The root of seven.
Update: Here’s Python code to validate the mnemonics above.
from math import sqrt def validate(line, num): digits = [str(len(w)) for w in line.split()] x = int("".join(digits)) / 1000 assert(x == round(sqrt(num),3)) validate("I wish I knew", 2) validate("O charmed was he", 3) validate("So we now strive", 5) validate("It rhymes with heaven", 7)
Related post: Numbers worth memorizing.