# Equations - HP -28S Manual

Expressions as Functions. In chapter 4 you created the expression
RTOT and, using the Solver, assigned values to the variables and then
evaluated RTOT to calculate the desired result. In this case the
expression acted as a function which, given the input values,
produced a result.
Expressions as Implicit Equations. In chapter 8 you used the
Solver to find the numerical zero of an expression-that is, the
numerical value of the independent variable for which the expression
has value O. In chapter 9 you used QUAD to find a symbolic zero-
that is, an expression which, substituted for the independent variable,
would give the original expression the value O.
In both cases the expression f(x) acts like the equation f(x)
=
0,
because the zero of the expression is the same as the root of the
equation.
Equations
Equations comprise two expressions related by an equals sign (=). In
mathematics there are two uses for the equals sign:
• To indicate a proposition, such as
u
x
2
=
4
U
or
u
x
2
+
y2
=
1.
u
Here
the equation holds only for some values of the variables .
• To indicate an identity or definition, such as usin 2x
=
2 sin x cos
XU
or
u
y
=
3x
2
+
2x
+
5." Here the equation holds for all values of
the variables.
On the HP-28S, equations are used for propositions only; to make a
definition such as fly
=
3x
2
+
2x
+
5," the expression
'3l>::·····2+2l>::+5'
is stored in a variable named
Y.
In "Time Value of Money" on page 103, both TVM and SPPV are
expressed mathematically as equations. The TVM equation, which
holds only for certain values of its variables, is entered as an equation;
but SPPV, whose value is defined by the value of its variables, is
created as a variable.
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