Quarter Square Multiplication:

a*b = ((a+b)² -(a-b)²) / 4

Wikipedia:

Multiplication algorithm - Wikipedia

From the book:

Instrument Engineers’ Handbook, Volume Two: Process Control and Optimization

edited by Bela G. Liptak

Instrument Engineers’ Handbook, Volume Two: Process Control and Optimization - Google Books

Quarter Square Multiplication in Analog Computing:

The Quarter square multiplier technique has also benefitted 8-bit systems that do not have any support for a hardware multiplier. Charles Putney implemented this for the 6502

Fastest 6502 Multiplication Yet

From:

Apple Assembly Line - V6N6 - Mar 1986 (txbobsc.com)

Charles Putney:

Charles Putney - Professional Profile - CodeProject

6502