Incidentally, the example above is called a point of undulation. I find that the graph of the second derivate (purple in this graph) nicely shows how the second derivative can be zero without a change of sign.

Rob is correct. Some bright spark at Casio thought it would be a good idea to confuse students all over the world by truncating. Use OPTN -> Edit (F1) while the value is highlighted to see a more precise value.

OCR people have to use this. I do not like it because I can't draw a man in a cave and say the curve is concave.

I'd say it's about as useful as this:

I'm a big fan of 'blobs' for things like this (and various other topics, like chain rule).

As others have said, it seems to be a hangover from when tables were used.

Edexcel thankfully seem to have given up on using Z unnecessarily now in mark schemes. In the images here you can see they were clinging onto it in 2018 (1st MS), but by 2021 they'd finally let go (2nd MS).

Your examples here are analogous to claiming that all computer graphics aren't very good and then using an Amiga 500 to justify your claim. I just ran both these queries through gpt 01 and it gave flawless answers.

It's also bizarre that they use f and g when hardly anyone teaches it that way. It's not like they couldn't use u and v because OCR do in their formula book.