The plural of formula is formulae or formulas. Formulae contain equals signs, as do equations, so it can sometimes be difficult to tell the difference between an equation and a formula.

It is strange that no reference is given to the work of Kronecker and others on equations which do admit of algebraic solution. The Theory of Equations: with an Introduction to the Theory of ...

He revolutionised algebra and his seminal works ... In particular, al-Khwārizmī developed a formula for systematically solving quadratic equations by using completion and balancing to reduce ...

Substitution means putting numbers in place of letters to calculate the value of an expression ... of 20°C is the same as 68°F using the formula.