HP 50g Graphing Calculator - Manual (Page 227)

Page 6-21
Example 3 – Flow in a pipe
You may want to create a separate sub-directory (PIPES) to try this example.
The main equation governing flow in a pipe is, of course, the Darcy-Weisbach
equation. Thus, type in the following equation into EQ:
Also, enter the following variables (f, A, V, Re):
In this case we stored the main equation (Darcy-Weisbach equation) into EQ,
and then replaced several of its variables by other expressions through the
definition of variables f, A, V, and Re. To see the combined equation, use
EVAL(EQ). In this example we changed the display setting so that we can see
the entire equation in the screen:
Thus, the equation we are solving, after combining the different variables in the
directory, is: