George Garside

Equation Solve for X - fx-991EX

Solving equations for variable X is possible with the equation solver in the fx-991ES PLUS calculator. Once the equation has been entered, the calculator uses Newton-Raphson numerical method to solve the equation.

Casio FX-991ES PLUS Equation Solve Calculator Display Screenshot
Casio FX-991ES PLUS Equation Solve Calculator Tutorial Animation

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.

Buy a Casio fx-991EX calculator on Amazon using this affiliate link to help support this site.

Advertisements
Advertisements

85 responses

Leave a Reply to reanakua Cancel reply

Your email address will not be published. Required fields are marked *

  1. ---
    82

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

    Reply
    1. joebiden
      3

      Do maths

      Reply
  2. klmmm
    17

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

    Reply
  3. abcde
    9

    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?

    Reply
    1. grgarside
      3

      The previous value of x is shown as the starting point for the calculation. After pressing Solve and entering a starting point (or using the existing starting point), you need to press equals. Please follow the instructions closely and let me know if you’re still having issues.

      Reply
      1. Acerage fx991 plus user
        2

        When I press = after calc it just returns a syntax error. Fx991 plus

        Reply
      2. shinobi
        1

        How do you enter a starting point ? ive been stuck in this step for weeks

        Reply
        1. JR
          3

          If your equation has more than one solution enter the equation as y = equation, then solve for y = 0 and update the starting x value.
          For example, y = x^2 - 10x - 36, "Shift Solve", y = 0, x = -7 will return x_1 = -2.8 while x = 15 will return x_2 = 12.8.

          Reply
          1. Shubham Sahu
            3

            Why you took x=-7 and 15????

            Reply
        2. Khanyi
          1

          I would also like to know the answer to this question.

          Reply
  4. Aryan Sharma
    7

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

    Reply
    1. Stephanie
      9

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

      Reply
  5. David Nyan
    6

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

    Reply
  6. Zoeyy
    5

    x^2-6x=0

    Reply
  7. Jinny
    4

    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

    Reply
  8. unnati
    4

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

    Reply
  9. good day
    2

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

    Thank you

    Reply
  10. j
    2

    How to solve quadratic equation which it has 2 answers?

    Reply
    1. Joseph Malone
      2

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

      Reply
  11. 12345
    2

    It says "can't solve" what to do?

    Reply
  12. Sandile
    2

    (-2y-1)^2-7-y^2=y

    After I press shift solve it says variable error

    Reply
    1. grgarside
      1

      You can’t use Y, you must use X.

      Reply
  13. Benister k Baiden
    2

    Tnks I want to learn how to punch for calculator

    Reply
  14. Wendy
    2

    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?

    Reply
  15. Aidan
    2

    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.

    Reply
  16. Mong
    2

    How to fix: when I press the division button"÷", "4÷" appears.

    Reply
  17. help
    2

    I have a problem when I shift solve an equation with atleast 3 variables it replace the value of the other variable, for example i have a b and x instead on finding x it finds the value of b and replaces its value how can I fix it classwiz 991

    Reply
  18. yasmin
    2

    i have a problem when it show the number it show a wrong answer for example it show 9999960 instead of 120 how can i fix it?

    Reply
  19. Richard Akesseh
    2

    Very good

    Reply
  20. KATY GWENDOLYN
    2

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

    Reply
    1. grgarside
      2

      Follow the instructions on the equation solve page.

      Reply
  21. ahmed nauman
    2

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

    Reply
  22. Priyamvada Arte
    2

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

    Reply
    1. omarpit26
      2

      calculate the fraction and then click on the SD button just above DEL

      Reply
  23. mhdshbzshrf
    2

    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!

    Reply
    1. Justin
      2

      After the solve for X is shown, press = . It may take some time but the answer will be shown later.

      Reply
  24. Amad
    1

    "Xsinx + cos x" how to solve it?

    Reply
  25. Chothiru Wagachchi
    1

    How to get M in the display?

    Reply
    1. grgarside
      2

      I'm not sure what you mean, can you expand your question?

      Reply
    2. Lorie Mae Fabia Bautista
      1

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

      Reply
  26. Piyush Singh
    1

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

    Reply
  27. Dhruvi Patel
    1

    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

    Reply
  28. محمد عمران
    1

    thanks it works for me.

    Reply
  29. Arslan Iftikhar
    1

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

    Reply
    1. grgarside
      1

      Can you be more specific with what your problem is? Do the instructions not work?

      Reply
      1. Arslan Iftikhar
        1

        we have to find value of x after solving this equation we get two value of x

        Reply
        1. grgarside
          1

          What doesn’t work?

          Reply
          1. Dwaipayan Roy
            0

            I suppose the calculator gives only one value which isn't sufficient for the problem

            Reply
  30. uuuu
    1

    how can I write sine^2(x)-x^2+1?

    Reply
  31. Charbel Chahine
    1

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

    Reply
  32. Student
    0

    What is L-R?

    Reply
    1. grgarside
      2

      The difference between the left expression and the right expression is shown as the solution to L–R.

      Reply
  33. gauti
    0

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

    Reply
    1. grgarside
      0

      Yes, if you change the starting point of Newton-Raphson as appropriate such that the other x is found.

      Reply
      1. Kate
        0

        How do I know what Newton-Raphson value to change it to? For example, if I'm solving a quadratic equation, there should be two x values, however I'm only able to get one???

        Reply
        1. grgarside
          0

          You have to think about how Newton-Raphson will be able to find the other value. It's easier if you know how the method works. If not, I guess you can just try a number far from the solution you've found but it might take too long for the calculator (or be in the wrong direction).

          Reply
          1. Sean
            1

            I can find the first solution. How do you input a start point for "x" on the calculator to find the second value?

            Reply
  34. Pre uni
    0

    What if my calculator is an fx-82AU PLUS II 2nd edition

    Reply
  35. Jerry W.
    0

    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.

    Reply
  36. Abdul Razak
    0

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

    Been at it for hours now

    Reply
    1. grgarside
      0

      Simply enter your question with an x on both sides of the =. Make sure you choose a suitable starting value.

      Reply
  37. Dennis Miller
    0

    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?

    Reply
    1. grgarside
      0

      What values for the starting point have you tried?

      Reply
      1. Khanyi
        0

        How do you set starting points on the calculator?

        Reply
  38. GEORGE
    0

    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.

    /Regards

    Reply
  39. Iryna Dymkar
    0

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

    Reply
    1. grgarside
      0

      Are you following the instructions accurately? Press equals at the "Solve for X" screen to calculate the solution. Make sure you're not confusing the starting point value which you can set with the actual solution to the equation obtained by pressing equals.

      Reply
      1. Iryna Dymkar
        1

        https://uploads.disquscdn.com/images/787851842fc70ba2fda3adfb822fb62bf2238bb6da97af4dd287aab456078e27.jpg https://uploads.disquscdn.com/images/0915aa93bcbe7945b42a345e3f08ce4ca754601102cb9d7fc6f453fdfd14bce4.jpg

        Reply
        1. grgarside
          1

          Yes… That’s not the solution, that’s the stating point for the numerical method. At the second screen, type your starting point and press equals, as I said.

          Reply
          1. Iryna Dymkar
            1

            How to return to the initial settings?

            Reply
            1. grgarside
              0

              Shift, 9, 3, =, AC.

              Reply
          2. Iryna Dymkar
            0

            I think it's some setting problem. Equation solver worked well. But after my daughter played with the calculator, equation solver crashed.

            Reply
  40. Kay
    0

    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?

    Reply
  41. Arjun M
    0

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

    Reply
  42. Diya
    0

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

    cant solve why ?

    Reply
  43. Deepali
    0

    20×5÷18(t)+1÷2×0.9×t^2=80+x

    How to solve the above equation in 991ES Calci?

    Reply
  44. Paige
    0

    No matter what function I enter, my solver always says X=1.666591966x10^(-5) ... anyway I can fix it?

    Reply
    1. Bobby
      0

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

      Reply
  45. VAIBHAV GUPTA
    0

    My CalC does nothing after shift solve

    Reply
    1. A.Bharat Karki
      1

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

      Reply
      1. reanakua
        1

        it should be in 1-Comp mode

        i wasnt able to use it with 2-cmplx

        Reply
    2. grgarside
      0

      What's displayed on the screen before and after the shift-solve function is activated? You're welcome to post photos.

      Reply
  46. Benister k Baiden
    0

    Tnks it helps me alot

    Reply
    1. Sylvester Jiminbee
      0

      Then explain how u start for me

      Reply
  47. MOHAN
    0

    write value of x in 24x^11+32x^7+15x^2+30x+21=0

    Reply