HP 35s User Manual page 243

A function programmed as an equation is usually included as an
expression specifying the integrand — though it can be any type of
equation. If you want the equation to prompt for variable values instead of
including INPUT instructions, make sure flag 11 is set.
4. End the program with a RTN. Program execution should end with the value of
the function in the X–register.
Example: Program Using Equation.
The sine integral function in the example in chapter 8 is
This function can be evaluated by integrating a program that defines the integrand:
  
 
 
Checksum and length of program: D57E 17
Enter this program and integrate the sine integral function with respect to x from 0 to
2 (t = 2).
Keys:
(In RPN mode)
9
(2)
 S

 X
9
(1)
sin
t
∫
Si(t) (
0
Defines the function.
The function as an expression. (Checksum and length:
0EE0 8).
Ends the subroutine
Display:
_

∫ 


Solving and Integrating Programs
x
) dx
x
Description:
Selects Radians mode.
Selects label S as the integrand.
Enters lower and upper limits of
integration.
Integrates function from 0 to 2;
displays result.
Restores Degrees mode.
15-9