HP 35s Scientific Calculator - Manual (Page 112)

6-10 Entering and Evaluating Equations
Expressions. The equation does not contain an "=". For example, x 3 + 1
is an expression.
When you're calculating with an equation, you might use any type of equation —
although the type can affect how it's evaluated. When you're solving a problem for
an unknown variable, you'll probably use an equality or assignment. When you're
integrating a function, you'll probably use an expression.
Evaluating Equations
One of the most useful characteristics of equations is their ability to be evaluated —
to generate numeric values. This is what enables you to calculate a result from an
equation. (It also enables you to solve and integrate equations, as described in
chapters 7 and 8).
Because many equations have two sides separated by "=", the basic value of an
equation is the difference between the values of the two sides. For this calculation,
"=" in an equation is essentially treated as "–". The value is a measure of how well
the equation balances.
The HP 35s has two keys for evaluating equations:  and  . Their
actions differ only in how they evaluate assignment equations:
 returns the value of the equation, regardless of the type of equation.
 returns the value of the equation — unless it's an assignment–type
equation. For an assignment equation,  returns the value of the right
side only, and also "enters" that value into the variable on the left side — it
stores the value in the variable.
The following table shows the two ways to evaluate equations.