<expression>[=<expression>]
• Process an expression or equation.

Expressions consist of numbers, variables (a-z, A-Z), operators (+, -, /), parentheses, and underscores (_) for the last result.  Multiplication is implied for two adjacent operands.

`e.g.   -9.8(y-Y)/(x-X)+2b`

Entering an expression causes it to be evaluated immediately.  If the expression contains any undefined variables, an error message is displayed.

Two expressions joined by an equals sign (=) form an equation.  Equations in one variable are solved for the variable; its value is stored as the result of the right-hand side expression.  Equations in two or more variables are always stored in the equation list.  Those "multivariate" equations automatically solve for their unknown variable when the other variable(s) are known.

`e.g.   x=4             Enter simple definition of "x".       1+5(x+2)=9x-5   Define "x" using a more complex equation.       y=mx+b          Enter and store the equation of a line.`
See also:  list, info, clear, remove, solve, substitute, eliminate, constant