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Casio fx-991 DEX Operation

Casio fx-991 DEX - General operation and getting started

The CASIO fx-991 DEX calculator has a large number of functions that can only be mapped on the keyboard by assigning double or triple keys. To select the second or third level from the first key level, there are the SHIFT (yellow) and ALPHA (red) keys.

Casio 991 DEX Overview
The most important keys for the start
Key Shift Alpha 991 DEX
Other key assignments: SHIFT / ALPHA
Key ON Casio 991 DEX
The MENU-SETUP and ON buttons
Button AC / OFF

Casio fx-991 EN X - The main menu

Casio fx-991 DEX Main Menu

The button at the top right under the text MENU SETUP always takes you back to the main menu of the Casio fx 991 DEX.

Back to the main menu with the button MENU / SETUP

Use the arrow keys in the middle under the display or the respective number to jump to the individual items in the calculator in the menu.

Arrow keys / key ring for navigation
Casio 991 DEX Display
Display - MENU / SETUP

The main menu of the CASIO FX 991 DEX has 13 sub-items, from 1 - 9 as well as A, B, C, D.

1: Calculations: General calculation mode

2: Complex numbers

3: Base N: Calculate in different number systems in base N, e.g. two's system, hexadecimal system, etc.

4: Matrices: Calculations with matrices

5: Vector calculus

6: Statistics

7: Distribution function: probability calculation

8: Spreadsheet: Calculating with tables

9: Tables: Value table, create function values in table

A: Equation/function: Solveequations numerically

B: Inequalities: Calculate inequalities

C: Check calculations: Check calculations

D: Ratios: Ratio equations / rule of three

Menu 1 991 DEX
Main menu: 1 - 8
Menu 9 991 DEX Casio
2nd page in the main menu: 9, A, B, C, D

Casio fx-991 DEX - Settings and SETUP

Casio 991 EN X Setup:

By pressing the SHIFT key and then the MENU key, you can access the settings(setup) of the calculator. However, this ONLY works in the calculation window (main menu 1).

With SHIFT MENU you enter the window for the general settings of the calculator. There are a total of 4 windows, each with 4 setting options. You can jump between the individual windows with the down arrow key.

You can also see the current position of the window by the black line on the right side of the respective window!

Arrow key "down
Window 1 - Settings
Settings - Window 2
Window - Settings
Settings - Window 4

Casio 991 EN X - The angle measure: degree measure / radian measure

Casio 991 DEX: In geometry and trigonometry lessons, it is important to set the angle measure when calculating with angles and angle functions.

In the 1st window of the Casio 991 DEX calculator, we select item 2: Angle unit for setting the angle measure. We get the following selection, see right.

1: Degree measure

for angle calculations in the known \(0° - 360° \) dimension

2: radian measure

\(360° = 2\pi, 180° = \pi, 90° = \frac{\pi}{2}\) etc.

3: Geodetic angle measure

 (GON), we do not need!

Window 1 - SETUP: select 2: Angle unit
Now select degree measure (D) and 2: radian measure (R)

Casio fx 991 DEX - The fraction display

If we calculate tasks with the Casio fx 991 DE X in fraction calculation or simply divide, we automatically get results in fraction representation. In the SETUP of the calculator, you can choose between pure fraction representation and mixed fraction representation (i.e. the integer part is in front of the fraction). To make this setting, we select the second window in SETUP and now 1: Fraction result.


1: ab/c

mixed Fraction representation

Ein Beispiel: \(15:4 = 3\frac{3}{4} \)


2: d/c

pure Fractional representation

Ein Beispiel: \( 15:4 = \frac{15}{4} \)

SETUP: 1: Break result
Casio 991 dex setup fraction
1: mixed fraction representation / 2: pure fraction representation

Casio fx-991 EN X - Decimal Display / Decimal Prefixes

In the first window of the settings of the Casio fx-991 EN X calculator, the following can be selected under

4: Decimal prefixes can be set. This can be confusing in some circumstances. These are abbreviations for thousands(k for kilo, M for mega, etc.)

1: One decimal prefix switched on

\( 2000:10 = 2k \)

k for kilo, factor 1000


2: Off Decimal prefixes switched off

\( 2000:10 = 2000 \)

4: Decimal prefixes
Enable decimal prefixes
Decimal prefixes 1: Switch on / 2: Switch off

Casio fx 991 DEX - Basic Inputs / Invoices

General calculations are performed in the calculation window of the Casio fx-991 DEX calculator. This appears by default when the calculator is switched on. If you are in another window, you can return to this point via the main menu and 1: Calculations.

Entries and calculations are concluded with the "=" key.

With the arrow keys left and right you can move the cursor in a calculation expression and make changes at the corresponding position.

Press the DEL key and you delete the character to the left of the current position.

Pressing the AC key deletes the entire entry.

The up and down arrow keys scroll through the previously performed calculations and you can view them again in the display.

Casio fx 991 DE X - The answer memory

The answer memory (ANS) of the Casio fx 991 DE X stores the result of the last calculation.


We calculate

\( 100:50 = 2 \)

Now we just press +10, it will appear: Ans+10.

The result of the previous calculation (2) is taken from the answer memory and 10 is added:

\( 2 + 10 = 12 \)

The result of the calculation is stored in the response memory.
Casio fx 991 of the answer memory 1
Read the result from the answer memory with "Ans" and continue calculating

Casio fx-991 DEX - Exercises on the general settings and the SETUP

Casio fx-991 EN X - Exercises - Carry out the tasks on the calculator first and only then check whether everything was correct!

1. exercise

Calculate the following tasks with the Casio fx-991 DEX calculator and, if necessary, select the correct settings in SETUP so that exactly the result given here is displayed.

a)   \( 30 : 4 = \frac{15}{2}\)

b)   \( 30 : 4 = 7 \frac{1}{2}\)

c) \( 30000 : 30 = 1k \)

Exercise 2

Enter the following numbers and correct the given digits!(Do NOT finish the first input with "="!)

a) \( 667789 : 9\)

NEW: \( 66789 : 9 \)

b) \( \sin(39) \)
new: \( \sin(30)\)

c) \( \sqrt{125}\)  

neu: \( \sqrt{121} \)

Exercise 3 - Audio

Perform the following calculations exactly in the given steps!





Do not check the solutions on the Casio fx-991 DEX until you have really tried to solve the problem yourself!

Task a)

Step 1: select in the setup fraction result - 2 d/c pure fraction display

Casio 991 dex setup fraction

Step 2: Enter the arithmetic expression.

Fractions Task 1a


Task b)

Step 1: Select mixed fraction representation 1: ab/c in the setup.

Casio 991 dex setup fraction

Step 2: Enter the arithmetic expression now.

Fractions Task 1b


Task c)

Step 1: Turn on decimal prefixes!

Enable decimal prefixes

Step 2: Now enter the arithmetic expression.

Exercise decimal prefixes with Casio 991 DEX