HP 48gII - Manual (Page 178)

Page 5-7
In ALG mode, substitution of more than one variable is possible as illustrated
in the following example (shown before and after pressing `)
In RPN mode, it is also possible to substitute more than one variable at a time,
as illustrated in the example below. Recall that RPN mode uses a list of
variable names and values for substitution.
A different approach to substitution consists in defining the substitution
expressions in calculator variables and placing the name of the variables in
the original expression. For example, in ALG mode, store the following
variables:
Then, enter the expression A+B:
The last expression entered is automatically evaluated after pressing the `
key, producing the result shown above.
Operations with transcendental functions
The calculator offers a number of functions that can be used to replace
expressions containing logarithmic, exponential, trigonometric, and