Calculate exp() and log() Without Multiplications
http://www.quinapalus.com/efunc.html
John Graham-Cumming: Squaring two digit numbers in your head
http://www.jgc.org/blog/2010/03/squaring-two-digit-numbers-in-your-head.html

x^2

Aims to provide both short and simple answers to the common recurring questions of novice programmers about floating-point numbers not 'adding up' correctly, and more in-depth information about how IEEE 754 floats work, when and how to use them correctly, and what to use instead when they are not appropriate.

Calculation by hand