Not perfect, but class

Operation:
At the start you will find the top right under the three vertical dots icon settings (- Load, Save - Save, Feedback - provide comments / questions, rate it - rate Settings - Settings -> this divided into general - in general, Calc - calculate, graph - graph drawing, About - about the program = version, here 2.3.87).

The question mark left of it leads to barren help function under "Online Help" it comes from in's Internet - without LAN not as good.

On the far left, next to the program icon, hidden under the three vertical bars the switchover Calc - calculate, graph - draw graphs and Table - Show table (s). Among them you will "Library" library for Constants - Constants, Functions - Functions and Expressions - (mathematical) expressions; within these three variants can be at the top of any back and forth. Unfortunately, the Library is used only for creating your own definitions (see also below in the text).

After you select an option (eg Calc) is found at the bottom left below the designated with the numbers 1, 2 and 3 rectangular buttons the three "workspace" said planes for simultaneous execution of different calculations.
On the bottom right side there are the symbols of three different virtual keyboards, labeled with 0 - 9, A - Z and a stylized keyboard. All these buttons after calling a keyboard also still appear directly above this, what a rapid back and forth allows - very good.
As with the new physical / real calculators is called for to experiment, but you should be able to get the first results quickly.

The following text I have (similar) posted as a comment on the criticism of Berolinus:
If constants should be pre-defined, you can not seem to see in the app unfortunately, which are possibly present. It probably remains currently the only way to find out about "Library" (accessible via the icon with the three vertical lines above left) "Add constant" itself to create and "Constants" / what you need. The same applies to the way on "Functions" and "Expressions".
Here is "RealCalc Scientific Calculator" offered by "Dwicinton App" actually better positioned (Note: there is also a dysfunctional "RealCalc" from another vendor that does not even mastered the point-on-wave control).

Unfortunately, only rudimentary, built-in help function of Matlab program is context-sensitive - after all, but you can jump from here to the online help (help DOT DOT mathlab us); on this page at the bottom you can help to download as a PDF file: "Graphing_Calculator_by_Mathlab.pdf" - there is on page 108 in "Chapter 6: Library" unexpectedly not dealt with in section "6.1 Constants", which constants (or not) are integrated, but only explains, adding how such self-defined and.

After all seems Mathlabs quite helpful providers to be (as can be seen on Google Play), which in "Post Image DOT org" turns off his answers to specific customer issues as images in PNG format (but the search function does not seem to be useful - or must be registered there) - it's definitely good if you know the image name and the full file path. Criticism or questions within the Amazon service is presently being probably still ignored by Mathlab, therefore it is advisable to look at Google play.

There, the app costs the 4.49 (Amazon normal 3.99 - for me just free); a version number is not displayed because of different equipment versions there when updated March 14, 2015 called - as new features are listed, the Amazon version seems to be up to date.
On my Kindle Fire HDX 7 with Fire OS 4.5.3 the app is running not only absolutely liquid, but also looks great from (to test the 3D display - super).