A literal equation is one in which some or all of the constants are represented by letters.
Example 65:The following is a literal equation:

A formula is a literal equation that relates two or more mathematical or physical quantities. These are the equations that describe the workings of the physical world. In Chap. 1 we substituted into formulas. Here we solve formulas or other literal equations for one of its quantities.
When we solve a literal equation or formula, we cannot, of course, get a numerical answer, as we could with a numerical equation. Our object here is to isolate one of the letters on one side of the equal sign. We "solve for" one of the literal quantities.
Example 66:Solve for x:
Solution: Our goal is to isolate x on one side of the equation. Removing parentheses, we obtain
Subtracting bx and then ab will place all of the x terms on one side of the equation.
Factoring to isolate x yields
Dividing by (a − b), where a ≠ b, gives us Check: We substitute our answer into our original equation. 
We solve literal equations by calculator in the same way we solved numerical equations. Select ...
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