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What can I say, the Casio FX991MS is a great calculator. It can be used for regular functions such as adding, multiplying, subtracting and dividing numbers. The more advanced features include squaring, cubing, square rooting, finding areas of circles etc.
The calculator also has second functions, this is simply the name given to buttons with two or more functions. These buttons are located on the upper area of the calculator. The first and main function of the buttons are accessed via a simple tap of the button. Accessing the other function(s) of a button involves a little bit more. Other functions of buttons are accessed by pressing the SHIFT button which is located on the top left of the calculator. Once the SHIFT button has been pressed you will be able to access alternate functions of buttons that allow you to.
Most of the buttons have 2 main functions, but some have a third option which allows you to bring up certain letters of the alphabet. To bring up the certain letters al you have to do is press the ALPHA button located on the top right and to the left of the SHIFT button. Once you have pressed the ALPHA button you will see that some buttons have letters above them, indicating what letter will appear after that particular button is pressed, but only after the ALPHA button has been pressed. I don't know the reason behind the letters, but to let you know the letters of the alphabet that can be accessed are: A,B,C,D,E,F,X,Y and M.
When you want to do a simple calculation, the numbers and functions you use will appear on the top section of the active area to let you know what buttons you have pressed. The great thing is that you can type in a very long calculation in the provided area, as the calculator allows a maximum of 78 characters of input. After you have typed in a very long calculation you can review it with the provided arrow keys, which allow you to navigate to the left or to the right of the screen. If you discover a mistake you can easily delete that mistake with the press of a single button. The button you use is called DEL and is one of the distinctive pink buttons on the right hand side and to the left of the AC button. The AC button is for getting rid of a whole calculation as well as shutting off the calculator. Shutting OFF the calculator is not as simple as pressing one button, to shut it OFF you have to press the SHIFT key and then the AC key.
There are also options of changing the calculator mode. To change the mode you simply press the MODE key, located on the top right hand side of the calculator and then choose any 3 provided options. The three options are COMP, SD, and REG and are accessed via the 1,2 and 3 number keys respectively. There is really no use for the 3 modes as I've tried using them and there seems to be no apparent change in the functionality of the calculator, as I thought there would be. The MODE key also has a second function allowing you to CLEAR MEMORY, CLEAR MODE and RESET ALL. The CLEAR MEMORY as the name suggests clears all memory on the calculator. The CLEAR MODE option returns the calculators MODE to the original settings, and the RESET ALL function as you guessed RESETS ALL. The three functions are accessed by first pressing the SHIFT function and then the MODE button. The three buttons are given the names Mcl (Memory Clear), Mode (Clear Mode) and All (Reset All) respectively.
The CASIO FX991MS initially runs via battery power and believe me it lasts for a considerable amount of time, as I've had the calculator for over 3 years without replacement of battery. And if the battery does run out you will not have anything to worry about as you can use your calculator with SOLAR POWER. The calculator has a small solar panel on the top right to absorb the solar energy in order to run. You can use the calculator in artificially lit areas as well as naturally lit areas. However, in artificially lit areas like regular rooms the calculator will not function as efficiently as in naturally lit areas. This is because natural light is far more powerful than artificial light. In lighted rooms the clearness of the viewing area will not be very good as the fairly small solar panel on the calculator will not be able to absorb sufficient amounts of light. You will encounter no such problems if used in clear Sunlight.
If you desire to replace the old batteries with new ones, you can do so buy unscrewing a part of the calculator located at the backside and at the top. It does not take AA batteries looking at the size of the battery area. I am not sure what batteries it takes as I have not yet replaced them. But I am certain that it takes small WATCH batteries, which are circular in shape and can be bought for a small price.
I purchased the CASIO FX991MS when it was fairly new and it cost me approximately £20. I bought mine from STAPLES when smaller and more compact calculators were just introduced. You may be able to get this very calculator for round about £10 now from most shops including STAPLES. Then again you can search online from places such as AMAZON MARKETPLACE and EBAY for better deals.
There are many, many more functions on the calculator, which are far too detailed to explain and would only appeal to the more advanced maths users. Overall the CASIO FX991MS is a great addition to any person who enjoys maths, it is very reliable and not very expensive regarding the range of functions.