
Making the Most of your Casio fx991ES Calculator
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A lot of people own an FX991ES, but I've found that very few of them realise how powerful this unassuming little calculator actually is. Here's a (not entirely comprehensive) guide to getting the best out of the Casio fx991ES. The guide assumes basic knowledge of how to get around the calculator (changing mode, changing from degrees to radians, memory etc). For more information, consult the manual.
Contents 
Mode 1: COMP
This is the mode you'll be using most of the time. It's probably the most versatile mode, giving you the options of definite calculus, calculation of functions and a rudimentary equation solver. Unless otherwise specified, this guide will be using MathIO, (Shift/MODE/1), Norm1 (Shift/MODE/8/1).
Definite Integration
It must be noted that integration can only be performed with respect to x. To perform definite integration, first press the button. This will bring up an integral sign, with blanks for the upper and lower limits and the integrand, and a 'dx' at the end. The cursor will be flashing for you to enter the integrand (in terms of x), using ALPHA  for example, . Once you have entered your integrand, input your upper and lower limits by scrolling up and down using the REPLAY/directional arrow buttons. Press [=], and it'll perform the integration, giving the answer as a fraction if appropriate.
Numerical Differentiation
Again, this can only be performed with respect to x. The calculator will, given f(x), calculate f'(x) for a particular value of x. Press SHIFT/ (that is, ). This will bring up something that looks a bit like this: . Enter f(x), and scroll right to enter the value of x at which you want to calculate the gradient. Press [=].
The CALC Button
This allows you to calculate the value of a function (of up to 7 variables) at a particular value of each variable. At a blank screen, enter your function in terms of the variables (using ALPHA)  e.g. . Press CALC. Prompts will appear for you to enter values of your variables. Don't worry if there's already a value displayed  this is just the most recent value that has been stored as that letter. Simply enter your value (in terms of pi, ln, log, fractions...) and press [=]. You will have to repeat this for each variable. The final press of [=] will calculate the value of the function.
The SOLVE Operation
This will solve most equations that you throw at it  with a few catches. The first major catch is that it only ever returns one solution  so if you're trying to find x with an equation in x^2, you're better off in equation mode, however multiple values can be found. The second catch is that you can only ever solve for one variable at a time  entering multiple variables will cause the calculator to prompt you to enter specific values for each of those variables save for x (or the one you've specified (by adding, say, ",Y" to the end of the equation). The third catch is that if your equation is rather complicated, it can take a very long time to solve accurately. The fourth catch is that it will only ever give you a solution as an integer or decimal.
I'll deal only with equations in terms of x, as doing it this way saves a few keypresses, and potentially a lot of valuable time. Enter the equation you want to solve, using ALPHA. It is not necessary for this to be in the form f(x) = 0. For example, . Then press SHIFT/CALC (SOLVE). The calculator will then display "Solve for X", along with the current value of X in memory (this can generally be ignored  see below). Press [=] again, and it'll come up with this:
What this means is that for , 3X + 2  6X = 0. "L  R" gives you an idea of the accuracy of the calculator's solution. In most cases , it'll be exact (assuming you have the sense to change recurring decimals into fractions).
If your equation has more than one solution there is a way in finding them take and you press solve, the screen you then see is if you type in a value the calculator will solve your equation giving you an answer nearest to the value you entered so for the example above, if you entered 80 as your value your answer will be 90, then when you solve it again and enter 250 as your value your answer will be 270
Summation
This allows you to calculate the sum of a series between two terms. The method for this is essentially the same as that for integration. To access the summation function, press SHIFT/ ( ).
Mode 2: CMPLX
In complex mode, the calculator is able to perform all expected calculations involving complex numbers. The ENG button (left of the open brackets button) becomes i. It is therefore possible to enter complex numbers in the form a + bi. Another option is to enter complex numbers in the modulusargument form. This can be done by first entering the modulus, then pressing Shift and the () button (), then entering the argument of the complex number in the correct format (degrees/radians/grads). It is possible to convert between the two forms from the complex menu (Shift2). This menu also allows you to find the complex conjugate of the complex number, as well as the argument. To find the modulus of a complex number, use the Abs function.
Mode 3: STAT
Statistics mode allows to input data in multiple forms and will give you the procedures required to calculate many values from the data sets (eg. standard deviation).
You can enter data with only one variable or two. After entering data, it is possible to carry out several different functions including variance and regulation calculations (which can be useful for Alevel maths).
Dunno fam this calculator is well sick
Mode 5: EQN
This is possibly the most useful of the fx991ES's 'extra' modes. You can use it to solve simultaneous equations of two or three variables, quadratics in the form , and cubics in the form .
Upon entering equation mode, you will be presented with the following options:
Simultaneous Equations
Options 1 and 2 are the simultaneous equation modes. Select option 1 if you have two unknown variables, or option 2 if you have three. Here I'll deal with two unknowns  the procedure for three is exactly the same. I'll use the simultaneous equations obtained from the solution to the following question:
"A man buys 3 fish and 2 portions of chips for £2.80 A woman buys 1 fish and 4 portions chips for £2.60 How much are the fish and how much are the chips?"
3x + 2y = 2.8 (equation 1),
x + 4y = 2.6 (equation 2),
where x is the price of 1 fish, and y is the price of 1 chip. The more mathematically able among you may already have seen that a fish costs £0.60, and a chip costs £0.50.
Upon selecting option 1, you'll be faced with a 3*2 matrix  the columns of which are labelled a, b, and c, the rows of which are named 1 and 2.
Enter equation 1's x coefficient, then press [=]. Do the same for the y coefficient and the constant on the RHS. Press [=] again, and do the same for equation 2. Pressing [=] again will show you first the solution for x, and another press shows the solution for y. So the keypresses are:
[3][=][2][=][2][.][8][=][1][=][4][=][2][.][6][=][=]
Quadratic and Cubic Equations
Options 3 and 4 are the polynomial modes  choose option 3 if you have a quadratic equation, and option 4 if you have a cubic equation. Note that they will provide you with all roots, both real and complex. Here I'll be dealing with a quadratic, , the roots of which are x = 1 and x = 0.5.
Enter the coefficients of x^2, x, and 1 in exactly the same way as has been outlined for simultaneous equations. Press [=] again and you will be shown the two roots of the equation, and . In the case of a repeated root, simply "X = " will be displayed.
Mode 6: MATRIX
Adding Matrices
To solve that press mode,6,1,1* and then enter values for your first matrix, then press on, then shift,4,2,2,1* then enter values again for your second matrix, then on again then press shift 4,3 then press add, then press shift 4,4, then press equals
*for 3x3
Scalar Multiplication
To do a scalar multiple of a matrix press mode 6,1,4* then enter your values of your matrix, then on, then press the number your multiplying by and then times, then press shift 4,3 and then equals
*for 2x3
Matrix multiplication
To multiply the above example press mode,6,1,4* and then enter values, then press on, then shift,4,2,2,2** then enter values again then on again then press shift 4,3 then press times, then press shift 4,4, then press equals
*for 2x3 **for 3x2
Transpose Matrix
To transpose a matrix press mode 6,1,2* then enter your values and press on, press shift 4,8 then shift 4,3 and then equals
*for 3x2
Inverse of Matrix
To find the inverse press mode 6,1,5* then enter your values and press on, then press shift 4,3, then press the button underneath the mode, then press equals
*for 2x2
Mode 7: TABLE
The table mode allows the generation of a table of numbers based on a function of X. This is a quick method of calculating several values for a function of X, as well as helping sketch graphs. Upon entering table mode, you'll be prompted to enter a function of X. Enter the value of X at which the table should begin, end, and the difference between each value and the table will be generated.
Mode 8: VECTOR
Throughout this section, I'll be using two 3D vectors, VctA and VctB  and respectively.
This mode allows you to perform calculations on 3D and 2D vectors  up to three at a time. Upon selecting vector mode, you'll be prompted to choose a vector memory slot to enter (VctA, VctB, or VctC). After choosing which memory slot you're going to use, you will be prompted to choose the dimensions of the vector (either 2 or 3). Now enter the values of your vector. Rather than using one column and three rows to indicate i, j and k, the fx991ES uses one row and three columns. Enter the i value (in the leftmost box), then press [=], which will prompt the calculator to scroll to the centre box, in which you enter the j value. Press [=] again, and it'll scroll to the righthand box, in which you enter the k value. Press AC, and you'll return to a blank screen.
To enter data for another vector, press SHIFT/5 (VECTOR) (which we'll now call 'Vector Options') and choose option 2 (Data). This will return you to the screen you were presented with when you first entered vector mode. Proceed from there as before, choosing a different memory slot.
Adding Vectors
We are going to add together vectors A and B. So we should be looking for an answer of . At a blank screen (AC), go into Vector Options and choose VctA (option 3). This will cause "VctA" to show up on the main screen. Now press [+]. Go into Vector Options again, and choose VctB (option 4). Press [=]. This will take you to VctAns (a fourth memory slot, uneditable by the user)  which indeed shows .
The Cross Product
This procedure is basically the same as that of adding vectors  only you press [] rather than [+]. Performing this for should yield .
The Dot Product
You might have been wondering what option 7 in Vector Options is for. It's for calculating the dot product of vectors.
Select vector A, reenter Vector Options, select Dot (option 7), go into Vector Options one more time and select vector B. Press [=]. You should see the following:
In contrast to convention the dot product has higher precedence than the vector product so you will sometimes need brackets around a vector product expression.
Magnitude
Press SHIFT/hyp (Abs), go into Vector Options, choose your vector, and press [=].
Unit Conversion
The Casio fx991ES can convert between different units of measurement, by first inputting the value to be converted, then pressing Shift and 8. A full list of conversions is given on the case of the calculator.
Scientific Constants
By pressing Shift then 7 in any mode except BaseN, it is possible to recall one of 40 constants stored on the calculator by inputting a number from 01 to 40. Although none are necessary for A level exams (they'll be given to you), the following may be helpful (though be aware that examinations may expect you to use specified rounded values):
 01  Mass of proton
 02  Mass of neutron
 03  Mass of electron
 06  Planck's constant
 17  Atomic mass unit
 24  Avogadro's constant
 27  Molar gas constant
 28  Speed of light
 35  Gravitational field strength
 38  Difference between Kelvin and Celsius
 39  Gravitational Constant
 40  Atmospheric pressure