It is labeled as "cos-1" on the calculator. But arcosine and inverse cosine are just two names for the same thing.
I don't know this model, but most calculators have one or more special function keys, which may be labelled Shift, Function, Inv, etc.; you have to press this key before you press the cosine key. This is correct, you have to press the "2ND" button (top left) and then "COS". You will notice that "COS-1" is written above the "COS" button- this is the function you want.
The 2nd function of the log button is the inverse log. Press 2nd log, which displays 10^( on the screen. Put a number after the opening parenthesis.
You can play Ti-84 games with the 84 keypad but I have yet to find games for the n-Spire CAS. So just download TI-84 games and use the 84 keypad.
There isn't a sec key on the TI-83 Plus. Secant is 1/cosine, which is what is used to find secant on a TI-83 Plus. For example, to find the secant of 4, enter 1/sec(4).
The TI-84 Plus Graphing Calculator could easily carry a student from the high school algebra to any upper level college math course such as trigonometry or even certain sciences such as physics. It has the sine, cosine, and tangent buttons to help a student find the information and with its speed 2.5x that of the TI 83, it can get to the solution even faster.
The TI-84 is a graphing calculater. It is mostly just an update to the TI-83 calculater.
Amazon has them for sale starting at £86.40
Yes, the TI-84 is allowed during the ACT.
Unfourtunately, it is not possible to expand with the TI-84. Only the TI-89 can expand polynomials.
Sorry, but no, to my knowledge there are no shells for the TI-84=CSE, and there probably will never be, the last descusions about being in 2006, and the TI-84+CSE is not compatible with the old TI-84 or even TI-84+SE programs. I am sorry to say but you will never have complicated games for this edition of the TI calculators but there are some simple games that don't need shells, such as tetris, worms etc:
If you are looking to download an app on your ti-84 or higher calculator, you should watch the "Quadratic Formula Program on the Ti-84" on youtube. Best of Luck!