HP 48g Graphing Calculator - Manual (Page 357)

Wrong units. A known or found variable may have units different
from those you assumed. These are global variables. If the variable
existed before this calculation, then its unit system (SI or English)
takes priority. To correct the units, either purge the variables before
solving the equation, or enter the specific units you want.
No units. If you’re not using units, your implied units majf not
be compatible among your variables or with the implied units of
constants or functions. The current angle mode sets the implied
units for angles.
Multiple roots. An equation may have multiple roots, and the
solver may have found an inappropriate one. Supply a guess for the
variable to focus the search in the appropriate range.
Wrong variable states. A known or unknown variable may not have
the appropriate state. A known variable should have a black menu
label, and an unknown variable should have a white label.
Inconsistent conditions. If you enter values that are mathematically
inconsistent for the equations, the application may give results that
satisfy some equations, but not all. This includes over-specifying
the problem, for which you enter values for more variables than
needed to define a physically realizable problem—the extra values
may create an impossible or illogical problem. (The solution satisfies
the equations the solver used, but the solver doesn’t try to verify
that the solution satisfies all of the equations.)
Not related. A variable may not be involved in the solution (no ss in
its menu label), so it’s not compatible with the variables that were
involved.
Wrong direction. The initial value of a variable may be leading the
root finder in the wrong direction. Supply a guess in the opposite
direction from a critical value—if negative values are valid, try one.
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The Equation Library 25-11