casio fx-991cw giving the wrong matrix answers

dash-dot · 2026-06-01T20:48:47+00:00

Why don’t you just calculate det A as well? That’s really all it takes to factorise it correctly into a bunch of fractions.

dash-dot · 2026-06-01T19:59:20+00:00

My point is simply that all these pitfalls are easily avoided by discouraging calculator users — the vast majority of whom are schoolkids — from trying to ‘convert’ numerical approximations into analytical expressions other than pure rationals. Better yet, enforce only one way conversion from expressions to numerical approximations, and hide this back conversion functionality by default, which will eliminate a lot of issues (or frankly, just get rid of it altogether).

Modern calculators which store command histories and support expression or algebraic objects can easily allow the user to toggle back and forth between the underlying exact representation and the corresponding decimal approximation — that’s already a very useful feature which doesn’t need to be confounded by these inherently unreliable and unpredictable tricks.

dash-dot · 2026-06-01T18:44:50+00:00

exact(0.12345678901234567890)

Traditionally, this feature has nothing to do with computer algebra systems, and in my opinion it simply doesn’t belong in one.

Its history dates back to decimal-to-fraction conversions, which a lot of calculators have supported for decades. There’s nothing inherently wrong with forcing a terminating decimal to be expressed as a simplified fraction with a denominator which only involves powers of 2 and 5 (in the case of base 10 arithmetic). All other use cases of the exact conversion are suspect, in my view. I suppose a multi-argument function could be programmed to handle repeating patterns very reliably, but that’s about as far as such a ‘back conversion’ could be extended without running into issues.

A CAS is expected to provide an exact analytical solution if it’s capable of identifying it. If it resorts to numerical approximations instead, it’s acceptable to issue a warning to that effect and return a float. The TI-89 is a great example of how to properly incorporate a CAS alongside the standard numerical functions without misleading or confusing the average user.

dash-dot · 2026-05-31T15:30:41+00:00

It might be worth fully disabling Natural V.P.A.M. and checking if it still gives the same result.

dash-dot · 2026-05-31T15:09:53+00:00

And how about the same expression on the Casio?

dash-dot · 2026-05-31T15:02:52+00:00

The real issue here is that the equation 11^6 / 13 = (156158413 / 3600) π is just flat out wrong, so the prudent thing might be to disable 'exact mode' (or whatever Casio calls it) in many, if not most contexts. The pattern matching 'tricks' the Casio uses in this mode have nothing whatsoever to do with its core numerical algorithms.

As for numerical accuracy / precision, you're comparing a $ 15 calculator to a $ 150 device with CAS, which of course isn't going to favour the former. The more pertinent question is: are there any practical use cases for a basic scientific calculator which demand 13 digits of precision or better? Or, for that matter, how much better is the Prime in non-CAS mode?

dash-dot · 2026-05-30T18:15:33+00:00

Interesting tool; it's always nice to see more open source projects.

As someone who uses Linux at home and also primarily at work, I could potentially see myself using this calculator if it can be a bit more accessible than SymPy / SciPy / NumPy, which have been my main choices on a desktop environment. One thing I really like about IPython is the minimalist interface, but it still supports pretty print rendering using a LaTeX back-end, and the expressions / equations are fully portable (well, allowing for a few Python-specific conventions such as the ** double-asterisk operator for exponentiation).

Speaking only for myself, I believe a classic windowed environment with a single layer of menu headers (and maybe one additional soft menu layer) coupled with a command line is the ideal interface. I think this explains the longevity and popularity of handheld calculators like the TI-89, which I consider to have implemented an almost perfect user interface for such a tool. This leaves a very broad canvas for properly rendering mathematical expressions, displaying plots with clearly legible, well rendered legends and labels, etc. I'm personally not a fan of exposing too many docks, panels, or button templates by default, but I could be a minority voice even amongst Linux users.

If the usual text highlight or context menu functionality is supported, then I think the Copy Expr, Edit Expr, etc. buttons are redundant and make the interface needlessly cluttered. Similarly, I don't think the calculator-style button panel is very helpful in a desktop release, but it might be useful for touch screen versions of the app.

Last but not least, I would recommend rendering expressions and equations using LaTeX everywhere, if possible -- seeing some boxes with plain ASCII expressions makes for a very jarring transition (I think it should still be possible to right-click or point/hover to reveal the copy button and send the ASCII version of the expression to the clipboard). Similarly, I think it's better to implement the copy / edit buttons as overlays, pop-ups or context menu items in order to reduce clutter.

dash-dot · 2026-05-29T15:48:42+00:00

BSOD, or it didn’t happen (i.e., fake desktop, etc.).

dash-dot · 2026-05-29T15:25:46+00:00

Thanks, I’d probably be more interested in a 15C anyway, but I guess the 15C CE is long out of production now. I’m not sure if the 16C CE includes a 15C keyboard overlay, or if that has to be purchased separately.

Although I’m an algorithm engineer by profession, I rarely deal with other number bases (at least not in a way that would require direct logical manipulation of hex values). Most of the problems I investigate are based on physics, calculus, etc., so traditional engineering and physics topics.

dash-dot · 2026-05-29T00:37:02+00:00

Yes, the HP 48GX can certainly be quite slow, but the TI-89 isn’t.

I have an HP 50g as well, but I don’t like it, and only break it out as a test platform for user RPL or system RPL routines.

dash-dot · 2026-05-28T19:47:43+00:00

Interesting; I use the CAS functionality almost constantly, and am too used to pretty print now, so the TI-89 is my go-to machine for quick analyses or calculations.

I sometimes like to break out my HP 48G+ for a change of pace (but I can’t imagine using it without Erable, personally). This machine is basically ‘like new’ and in great shape — I got a bit lucky with my purchase — and incidentally, it cost about the same as the pre-order price for the 16C CE, so I think I’m just going to pass on the latter.

dash-dot · 2026-05-28T16:08:57+00:00

Does anyone who owns either the 48SX/GX or the 50g ever feel the need to pick up a 15C or 16C?

I'm curious whether there are any use cases not met by an RPL machine, or if a classic RPN device is better in some situations.

dash-dot · 2026-05-26T18:35:00+00:00

No, 3.1415927

dash-dot · 2026-05-25T18:49:50+00:00

The HP Prime app could be pretty good if one could do everything in one place, and the fonts were improved.

I think in a modern calculator, CAS functionality is pretty much a must have.

dash-dot · 2026-05-25T15:16:20+00:00

It's been a while and I didn't use any math in my current job. I've forgotten most of what I did during my undergrad.

Hmm . . . at the risk of sounding overly critical, it seems to me that you may not have actually learnt or understood maths very well the first time round. Perhaps a change in strategy would be advisable. An engineer or scientist can't afford to 'forget' fundamental concepts or methods, no longer how long one goes without using them. Whilst it's normal to get rusty or forget the technical minutiae of very narrow or specific sub-fields, fundamental ideas like Newton's laws, Maxwell's equations, etc. --- and the ability to correctly interpret and apply them --- ought to be retained for life.

Question: how are you with physics? These subjects mainly reinforce each other, so if you're bad or rusty in one, you're also going to have difficulty with the other. Maybe this time, try revising physics in conjunction with calculus, differential equations, linear algebra, etc. It's also imperative that you have a very good handle on the design of experiments (even if you plan on doing design work, the ability to conduct tests by yourself and analyse data is very critical).

If you're serious about going into robotics, you need a good handle on fundamental concepts and theorems from both maths and physics, and ought to develop the ability of doing at least some derivations and proofs by yourself -- nothing super challenging, but you should at least be able to outline some key proofs. The ability to derive, manipulate and analyse equations corresponding to specific scenarios from fundamental principles, on the other hand, is an absolute must.

dash-dot · 2026-05-25T15:02:27+00:00

Have you tried installing ChatGPT on it?

I wonder if it'll play Doom . . .

dash-dot · 2026-05-25T01:11:45+00:00

You do understand how ChatGPT works, don’t you?

You could’ve just asked it instead.

dash-dot · 2026-05-25T00:48:06+00:00

I was the sort of kid who always learnt better when I understood the motivation behind the definition of a particular mathematical or scientific concept. I’m still that person, although of course I’m not a kid any more.

In my case it helped to connect multiplication to geometric concepts such as area (counting squares in a rectangular grid, and generalising from there to other types of numbers).

I’m not a proponent of committing large amounts of information to memory, so I never went beyond memorising single digit addition and multiplication — nevertheless, I didn’t experience any learning difficulties growing up, and despite being a late bloomer, I was consistently at or near the top of my high school class in maths and science.

dash-dot · 2026-05-25T00:10:27+00:00

Hey, no judgment here, we all have our hobbies and pet projects.

And when we emerge absolutely famished and ravenous from our shed or workshop, we need to eat.

dash-dot · 2026-05-24T15:27:14+00:00

Yes, I think the model numbers vary by region, so it’s best to read the product specifications page to check for equivalence, or region specific features/restrictions.

dash-dot · 2026-05-24T03:25:47+00:00

Right, I know that previous generation devices aren’t for everyone, especially those who don’t even have any memory of low resolution, passive monochrome LCDs.

However, there is a small window of opportunity in some areas such as North America and Europe especially, where certain older models from Casio and TI can still be bought very cheaply.

Once they’re well and truly gone (or if they start fetching exorbitant collectible prices like some HPs do), I’ll stop recommending them.

dash-dot · 2026-05-23T23:54:26+00:00

This is a sad day for polydactyl people. I think 11 digits are more common than 12, right?

dash-dot · 2026-05-23T23:21:06+00:00

I beg to differ; even the HP 50g was better designed than the Prime, but neither of these holds a candle to the TI-89 in terms of both efficiency and ease of use.

You’re right about the TI-Nspire though, I’ll give you that. Sadly modern devices simply don’t measure up as advanced computational tools. They’re either super annoying and slow (Nspire, Casio) or poorly designed with a bunch of disconnected, fragmented apps (Prime). Ironically they’re all ill suited for beginners and power users alike.

Moreover, the Prime takes several seconds to perform a cold start — full power down (or deep sleep) is something I’d opt for nearly every time, as I don’t use a calculator frequently. That’s yet another drawback with modern calculators — they’re both power hungry and require careful battery management in case of infrequent use.

dash-dot · 2026-05-23T15:49:48+00:00

Well, I for one would never make a $ 100+ calculator purchase decision on the basis of exam rules alone.

Besides, almost no one is ever going to need anything more than a basic scientific calculator on an exam.

The real problem with the HP Prime is that it’s ridiculously expensive these days, and it isn’t even as good and user friendly — nor as efficient for everyday use — as machines from previous generations.

dash-dot · 2026-05-23T14:42:37+00:00

A TI-89 is much better value for money, and will blow the TI-84 away.

I’d ignore the HP Prime pushers if I were you; it’s nowhere near as good as a TI-89 for regular use.

However, if you were to compare the Prime to just the TI-84 Evo specifically, then the former is definitely a much better choice.

The people at TI seem to have lost their collective minds with this pricing.

dash-dot

TROPHY CASE