Reddit reviews Texas Instruments TI-84 Plus Graphing Calculator, Black

https://apstudents.collegeboard.org/exam-policies-guidelines/calculator-policies

You might get good mileage out of it in HS, but since College Professors won't usually let you use them on tests or exams, in college, you're gonna want to learn to do it the hard way on paper.

I would NOT advocate for paying full-price for one.

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The cheapest nSpire I see on Amazon is in the $99 range, used.

https://www.amazon.com/s?k=nspire&ref=nb_sb_noss_2

But you can get a used TI-84+ in the $70 ballpark:

https://www.amazon.com/gp/offer-listing/B007WM3MW2

Or a perfectly legit used TI-83+ in the $30 ballpark:

https://www.amazon.com/gp/offer-listing/B00001N2QU

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The keystrokes and function operations are very similar across all of the TI calculators.
So when the textbook or homework says to press <this key> then <that key> to perform something specific, that should be pretty universal for all TI calculators.

In other words, I don't think you MUST have an nSpire, unless your teacher is being lazy and doesn't want to deal with any deviation from the nSpire key layout.

Some students mind's are completely blown when the <function> key isn't located in the exact same specific location as the textbook example.

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Two suggestions if you choose to purchase your own calculator:

You sure as hell don't want to have to buy a second one.

Buy a protective pouch/case for it like this:

https://www.amazon.com/LuckyNV-Protective-Instruments-Calculator-Accessories/dp/B075LH789Y/

You sure as hell don't want alkaline batteries to leak inside it and ruin it.

Some or most nSpires come with rechargeable battery packs.
But most TI-83 and TI-84 models use 4 x AAA batteries.

Lithium batteries should last for years and won't leak/corrode like an alkaline battery might.

https://www.amazon.com/Energizer-Lithium-Batteries-Battery-Ultimate/dp/B00003IEME/