equations
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Expression. Equations have equals signs.
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formula expression
Linear equations are a small minority of functions.
It is a linear expression in two variables. As an expression it cannot be solved. Furthermore, to solve equations in two variables you need at least two linear equations.
chemical equations
Numerical equations have only numbers and symbols, while algebraic equations have variables also.
A chemical equation is the graphic expression of a chemical reaction.
They both have variables.
formula of radius