HP or TI
While Texas Instruments (TI) graphing calculators are the more popular brand–and easier to acquire–some students may want to use a Hewlett-Packard (HP) graphing calculator. Because the TI calculator is more popular, HP 50g graphing calculator reviews must take into account the difference between the two.
What you'll find here, therefore, serves to highlight some of those differences in order to allow you to help those students using HP calculators.
Reverse Polish Notation (RPN)
Probably the biggest difference between TI and HP calculators is HP's use of RPN. The most recent model, the HP 50g, allows for a more TI-friendly "algebraic" interface, but the default is still RPN.
So what is RPN? RPN uses "stacks" to store multiple numerical values and expressions, which you can then use the operators to manipulate. For example, look at the keystrokes needed to find 2 + 2 in both types of interfaces:
TI – Algebraic
 [+]  [ENTER]
HP – RPN
 [ENTER]  [+]
The next example shows how to enter a complex expression, like the quadratic expression, in both interfaces:
TI – Algebraic
[ ( ] [–] [b] [+] [sqrt] [ ( ] [b] [x2] [–]  [×] [a] [×] [c] [ ) ] [ ) ] [÷] [ ( ]  [×] [a]
HP – RPN
[b] [–] [ENTER] [b] [x2]  [ENTER] [a] [×] [c] [×] [–] [sqrt] [+]  [ENTER] [a] [×] [÷]
As you can see, you don't need parenthesis in RPN. Each part of the expression is entered into its own stack and then those parts are put together.
Graphing on an HP is a bit more complicated than on a TI. I'll use the HP 50g as an example.
Step 1: Enter the function that you want to graph by holding down the left-shift key and selecting [Y=] ([F1]). Enter the function by using the "EDIT" option.
Step 2: Select the type of plotting you want to do, either 2D or 3D, by holding down the left-shift key and selecting [2D/3D] ([F4]). Select the type of plot you want to do, and verify that your function is in the space "EQ:".
Step 3: Define your plot window by holding down the left-shift key and selecting [WIN] ([F2]). Here you can define the range and domain of the plot.
Step 4: View the graph by holding down the left-shift key and selecting [GRAPH] ([F3]).
The reason for the separation of each step is that you can use the "Y=" function for more than graphing, such making a table or finding a solution.
The Equation Library
Probably the best feature of HP calculators is the equation library. On the HP 50g, you access the library by selecting [APPS] and scrolling to option 12. Select option 1 for the equation library, and again option 1 to view the library. The equations are grouped by category, and diagrams are included when appropriate. You can also use the feature to solve for specific variables.