Solving equations is possible with the equation solver in the fx-991ES PLUS or fx-991EX calculator’s shift-solve functionality. Once the equation has been entered, the calculator uses the Newton-Raphson numerical method to solve the equation. Because the equation solver uses a numerical method, it only works with equations with a single variable, but can find multiple values.

For example, I will solve the following equation for x:

  1. Enter the left expression using X. An X is inserted for variable that you wish to solve.
  2. Use = accessible through ALPHA CALC. This inserts an equals sign.
  3. Enter the right expression.
  4. shift-solve to enter equation solve by pressing SHIFT CALC. Solve for X will be shown on screen.
  5. If applicable, enter a number as the starting point. This Casio calculator uses Newton-Raphson numerical method solving for x. The current value of X is displayed for reference and will be used if no new value is provided.
  6. Press = to solve the equation. The calculator may take a few seconds.

The first value of x as found by the numerical method will be provided on the screen and is saved to the X variable. The difference between the left expression and the right expression is shown as the solution to L–R.

Caveats: As mentioned, this only works with equations with one variable. The variable can be used multiple times, as shown in this example, however you can’t use two or more variables such as x and y. If you wish to use more than one variable, you need to use the quadratic equation solve.

Furthermore, the Newton-Raphson numerical method only returns one value. To find another value, change the start point at step 4. For example, if there are two solutions, one positive and one negative, and you find the positive one, enter a negative number for the start point to find the negative answer.

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Leave a Reply

9 klmmm’s avatar

Is it possible to use variables to get an answer like this; 105+a+2a (press equals and then) 50+3a?

4 Aryan Sharma’s avatar

How to find multiple roots like in a cubic… Because like this it shows only one root.

1 Stephanie’s avatar

Go to mode. Then 5 (EQN). Then tap on 4 that shows a sample cubic equation. Then input numbers without the x. Done ✅

3 abcde’s avatar

i am having an issue where after presssing solve i get the value of x in my previous equation, not the present equation that i am solving. Can someone please help with this?

3 David Nyan’s avatar

Can I use sin and cos in the equations? And how can I use 2 variables with 2 equations?

1 ---’s avatar

My equation has 2 possible solutions. But the calculator is only displaying 1 of them. What should I do?

1 Wendy’s avatar

I have one question (It’s not in the FQA or Manual.)

How can I see the result with “pi*pi=” as a “pi^2” symbol rather than a decimal point?

0 Aidan’s avatar

Is there a way to get the fx-991EX to simplify a fraction that has a x on the top and bottom and not just solve it?

Ex: 10x^2-10x/2x-2

When i do this exact equation it gives me an answer, i want the simplified version.

0 Jerry W.’s avatar

Thanks loads for this very informative example. I recently purchased a Casio fx-991EX through Amazon because my previous calculator, a Casio FX-115 ( the for-runner of this one) died after 20 years. The documentation that comes with the Fx-991EX is not the best. I’ve been trying for several days to solve even the simplest quadratic equations with no success. They wanted me to enter the data using a “Coeffiicent editor” which was complex. I was on the verge of returning my calculator back to Amazon before my 30 days return period ran out. Then I got the idea of googling online to see if anyone else had a similar problem. I found your example ONLINE and no longer am planning to return my calculator.

0 Jinny’s avatar

When solving for a variable using SHIFT+SOLVE, how to I make the L-R equal to 0? because from time to time the L-R value is not 0 so there are inaccuracy in the answer

0 Zoeyy’s avatar


0 Benister k Baiden’s avatar

Tnks it helps me alot

0 Benister k Baiden’s avatar

Tnks I want to learn how to punch for calculator

0 Sandile’s avatar


After I press shift solve it says variable error

0 12345’s avatar

It says “can’t solve” what to do?

0 unnati’s avatar

y do u use another number (0,30,100,2,1) or = as the starting reference number??

0 Diya’s avatar

2\times 10^{5}=\sqrt{\frac{6.67\times 10^{-11}X}{6.7\times 10^{7}}}

cant solve why ?

0 Arjun M’s avatar

I am using 991EX CLASSWIZZ.When i press ‘=’,its showing ‘syntax error’.Could u help me with that

0 Kay’s avatar

If it’s okay to ask here. I am trying to solve this eqn 10/x = cosh(50/x) – 1 and it’s giving me an error. Any idea why?

0 j’s avatar

How to solve quadratic equation which it has 2 answers?

0 Joseph Malone’s avatar

You have to select mode, 5:equation, 3:aX^2+bnY+c= 0. “4:aX^3…” is for cubics.

0 Deepali’s avatar


How to solve the above equation in 991ES Calci?

0 GEORGE’s avatar

Is there any hack that can increase the time before the fx-991, or fx-115 times-out and turns itself off? Very frustrating because I lose my calculation history.


0 Dennis Miller’s avatar

Trying to solve for x in the equation 10x^9 – 9x^10 = 0.75. When I put the equation into an online equation solver, I get the solution where x = 1.063 and 0.9036. For the particular problem I am working, x is only defined between 0 and 1. Therefore, the answer is 0.9036. However, when I put the equation into my Casion fx-115ES Plus, it will only return the answer of 1.063. Is there a way I can get it to return the other value of x?

0 good day’s avatar

how to calculate a polynomial y=a+bx+cx2+dx3 with the casio fx-991za plus.

Thank you

0 Amad’s avatar

“Xsinx + cos x” how to solve it?

0 Charbel Chahine’s avatar

how to get both answer of the equation of second degres ? using solvw

0 Arslan Iftikhar’s avatar

√x^2-3x+36 -√x^2-3x+9 =3 how to solve this equation fx991es-plus

0 Iryna Dymkar’s avatar

My calculator always gives the same result when I do equation solver operations. What do I have to do?

0 mhdshbzshrf’s avatar

after typing my eqution and pressing ‘solve’ , why do they ask “Solve for X” ?

what should I do there ?

typing in any no. as a reply to the above (Solve for X) couldn’t always bring me the correct answer!

0 محمد عمران’s avatar

thanks it works for me.

0 ahmed nauman’s avatar

how to get an answer inthe foem of x like (x-2)(x-0)=x2-2x

0 VAIBHAV GUPTA’s avatar

My CalC does nothing after shift solve

0 A.Bharat Karki’s avatar

Wait for some time…My calculator takes some time for difficulat ones…

0 reanakua’s avatar

it should be in 1-Comp mode

i wasnt able to use it with 2-cmplx

0 Chothiru Wagachchi’s avatar

How to get M in the display?

0 Lorie Mae Fabia Bautista’s avatar

this is y question too, how do I get M on display? the little big M at the upper beside S and A

0 Abdul Razak’s avatar

How can i solve equations with variables on both sides on 991es plus

Been at it for hours now

0 Dhruvi Patel’s avatar

How can I simplify an equation, like simplify/add/subtract a polynomial. ex; -h^6 + 4h^5 -3h^4 + 2h^5 -9h^6. This would be simplified as 6h^5 – 10h^6 – 3h^4. How would you simplify that on a calculator


How can I solve quartiles using 991ES PLUS? Thanks.

0 gauti’s avatar

can this method give two values of x (if applicable) ?

0 Priyamvada Arte’s avatar

How do I solve problems like 111/88 on the calculator? It doesn’t calculate in decimals

0 omarpit26’s avatar

calculate the fraction and then click on the S<->D button just above DEL

0 Piyush Singh’s avatar

Can I solve Trigonometric equations like tan(x)=x ?

0 Paige’s avatar

No matter what function I enter, my solver always says X=1.666591966×10^(-5) … anyway I can fix it?

0 Bobby’s avatar

you have to hit “solve” and then the “=”. Then it has a loading thingie and then spits out an answer.