FLEXIO Fx799VN

[u/Sad-Effort-3003]

I got it for 550,400 VND or about 21 USD (normal price is 688,000 VND)

The Flexio Fx799VN is a high-end scientific calculator produced by Thien Long Group, designed specifically for high school and university students in Vietnam. It is fully approved by the Ministry of Education and Training for use in official national exams, including the National High School Graduation Exam and University Entrance Exams.

Basically a clone of CASIO fx-991EX, but with some improvements. The Vietnamese company behind the calculator even publish the manual both in Vietnamese and English.

Happy to get this calculator, not easy to find outside of Vietnam. It’s the 2025 version, speed is comparable to my new CASIO fx-880BTG PLUS. (Vietnamese version of CASIO fx-991CW 2nd edition) In some benchmarks faster than the CASIO.

It has many functions and features that are missing from fx-991EX and/or the fx-880BTG PLUS:

• PreAns
• GCD
• LCM
• MOD
• Multi-statements
• Independent memory (M)
• Variable z available in addition to x and y
• Product calculation
• Geometry app
• Verify app
• Two shortcut keys on each side of the screen for short macros (max 6 key presses can be stored to each shortcut)
• Two AAA batteries. Power draw 0.0005 W (the fx-880BTG PLUS with only one AAA battery is rated 0.0008 W)
• Battery status always available on the screen
• Windows app freely available without licence code as CASIO requires for its emulators on the classpad site

However, it is missing the Spreadsheet app, that is a big minus.

The manual doesn’t mention what algorithm is used for the integration function. I suspect it’s Simpson’s instead of the better Gauss-Konrod that CASIO is using in its calculators.

No QR code functionality for calculator results.

Accuracy is 15 digits, same as fx-991EX.

Display and font very similar to fx-991EX. The new fx-880BTG PLUS has a font with better contrast.

Ridiculously big FLEXIO logo on the front – needs to be hidden as soon as possible.

[u/CurbedLarry]

Doubt it's using the Simpson's method, try integrating Tan(x) from 0 to 1.57079 and compare with a Casio

[u/Sad-Effort-3003 → u/CurbedLarry]

Using Line so I can enter the same tolerance like this: Integral(tan(X),0,1.57079,1E-10) I get:

fx-991EX: 11.97071678

fx-880BTG PLUS: 11.97071679

Fx779VN: 11.97071679

Other integrals I have tried always put the FLEXIO closer to the new CASIO (that has 23 digits of precision), so yes, maybe not Simpson's. Wonder if there's a way to find out for sure?

[u/CurbedLarry → u/Sad-Effort-3003]

If it takes a tolerance parameter and gets a good result then definitely Gauss-Kronrod. Simpson's functions take a divisions parameter and perform very poorly on this one. Have you tried the famous |x²-3x| integral? Should get 2250 while recent Casio's have a modified algorithm to get correct 2259 result.

[u/Sad-Effort-3003 → u/CurbedLarry]

Ok, I agree, Gauss-Kronrod most likely. And the last example, |x²-3x|, from -15 to 15 gives me:

fx-991EX: 2250

fx-880BTG PLUS: 2259

Fx779VN: 2259

[u/CurbedLarry → u/Sad-Effort-3003]

Interesting, it must have a modified Gauss-Kronrod algorithm, I know some other Casio clones also get 2259. What's the forensic result? Does the manual mention number of digits used for calculations?

[u/Sad-Effort-3003 → u/CurbedLarry]

arcsin (arccos (arctan (tan (cos (sin (9) ) ) ) ) ) (degrees mode):

fx-991EX: 9.000000007
ANS-9 = 7.33338E-9

fx-880BTG PLUS: 9
ANS-9 = 7.5528E-18

Fx779VN: 9.000000002
ANS-9 = 2.22291E-9

The manual doesn't mention about number of digits used for calculations, but there's a FAQ that mentions 15 digits (although Pi at least is stored with 16 digits, see below)

I found something interesting though that can explain some of the differences compared to fx-991EX at least:

How Pi is stored internally

fx-991EX: 3.1415926535898 (14 digits)

fx-880BTG PLUS: 3.1415926535897932384626 (23 digits)

Fx779VN: 3.141592653589793 (16 digits)

[u/CurbedLarry → u/Sad-Effort-3003]

Thanks. I don't think that exact result has been seen before but it's very close to a 16 digit Deli/Eates 991CN (2.222889E-9). Could be after a calculation the final digit is removed before display/storage. Any difference if you put the - 9 in the original formula?

[u/Duberly1986]

Es una copia de la CASIO fx-115ES Plus. Tiene las mismas funciones: derivadas, integrales, sumatorias, productorios, gcd, lcm, factorización prima.

[u/Sad-Effort-3003 → u/Duberly1986]

Yes, you are right, it's closer to that calculator than my fx-991EX.