You can use them for POE, process of elimination.
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In the same way that you would solve equations because equivalent expressions are in effect equations
It may be possible to solve equations. Expressions cannot be solved until they are converted, with additional information, into equations or inequalities which may have solutions.
The answer depends on the nature of the equations.
Assume something (e.g. equations) using k then prove k+1 using k.
One can solve equations of motion by graph by taking readings of the point of interception.