I have a Casio fx 991ES and I'd like to have my answers in surd form instead of decimals how is that possible

Change the output display to MathIO. You may be able to display square roots only: so no cube roots, nor fourth...

Press the Shift/Mode key and set the Input/Output to MathIO.
Even that will not guarantee that the display will be in root form. Only square roots are possible.

Press the Shift/Mode key and set the Input/Output to MathIO.
Even that will not guarantee that the display will be in root form. Only square roots are possible.

Looks like <Shift><Mode> <down arrow> <1>

Give it a shot. I have attached the manual.

Good luck.

Paul
http://support.casio.com/pdf/004/fx-115ES_991ES_Eng.pdf

This is a good scientific calculator that can do a lot, but things in the realm of a CAS (Computer Algebra System) are beyond its capabilities.

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The capability of these calculators to handle irrational numbers in radical form is limited to square roots. Roots of index other than 2 are always displayed in their decimal representation.
If the calculator IO mode is set to MathIO, square roots will be displayed with the radical symbol. More complicated expressions with square roots might not be preserved and will be given as decimal numbers.

To convert a single answer from symbolic to decimal, press the S--D key (just above the DEL key). To have all answers come out in decimal format, press SHIFT MODE 2 to switch to the LineIO mode.

Press the S--D key to convert a value between decimal and symbolic forms. Keep in mind that the calculator has limits on the number of digits in the numerator and denominator.

The calculator can't identify that this is a surd unless you input it as a surd, ie as 3/(4*sqrt(5))= when it will then show the equivalent form 3*sqrt(5)/20 (obtained by multiplying top and bottom by sqrt(5)) and you can swap between the two forms using the S<>D button. Even adding the next 4 decimal places (hidden) won't allow the calculator to identify the underlying surd as this is simply too complcated a calculation.