Talk:equation

How does "equation" differ from "formula"? In grade school I was taught that a = b + c is an equation but b+c is a formula. Nowadays b+c is often called an expression, whereas a = b + c is often called both an equation and a formula.
 * Please see Talk:formula. \Mike 19:26, 2 May 2008 (UTC)

/ʒ/
How did equation come to have, when essentially all other -‹tion› words have ? Just a one-off chance voicing? Or is there an etymological justification I'm missing? — ˈzɪzɨvə 16:11, 2 May 2011 (UTC)