Every time I shop for back to school, I’m puzzled and frustrated by the request from the math teacher for a scientific or graphing calculator. I know the calculator is a necessary school tool. But it's also an expensive piece of technology. And I like to do cost comparisons before I plunk down a credit card for anything pricey, especially technology. Usually my son—or daughter, whomever I’m buying for—is in the store with me. So I inevitably pelt him with questions he can’t answer: “Can’t you use the one we bought last year? Isn’t there a $3 app that can do the same thing? Why do I have to get this model when there’s another I like better?” My son, always laconic, just shrugs and puts exactly the model the teacher requested into the cart. He isn’t paying for it. So when I spoke to the folks at Texas Instruments about the TI Nspire(tm) Apps for iPad ($30) that I covered in my back-to-school app story, I also took the opportunity to ask all my stupid calculator questions. It turns out my son is right. But it helps, somehow, to know why.

So here are my questions, with answers from Tom Reardon, 35-year math classroom educator and Texas Instruments Teaching with Technology Instructor.

Do I have to buy the exact calculator on the teacher’s list even if there is a cheaper—or fancier—one?

When it comes to graphing calculators, teachers choose a model for a specific reason (age-appropriate, latest features, etc.) so, yes. Be sure to purchase the exact model the teacher recommends. For example, I had parents buy their student a TI-89 or a TI-92, assuming the bigger number meant a better calculator. But it is very difficult for a student to learn to use a different calculator on his own. Most often, that other calculator was not appropriate to the ability level of the child.

How can I tell the difference between models? They all look the same.

It’s true that in the store the calculators are packaged so you can’t explore them. But you can research them before you go to the store. Texas Instruments’ website provides details on features and allows you to compare models. While shopping, be sure to think ahead about what tests your student will take in the coming years. Certain calculators are allowed on tests, but others aren’t. You want to ensure your child can use the calculator they are most familiar with when they take the test. TI gathers info, which it posts online, on which calculators are allowed.

Is there an app that’s cheaper and works just as well?

Yes and no. The TI Nspire(tm) Apps for iPad ($30) has all the functionality of the TI-Nspire CX handheld, but the app isn’t necessarily a replacement for a calculator. Gadgets like iPads, tablets, smartphones and laptops are not allowed on tests such as the SAT, ACT, AP, etc., or in some classrooms. The app is also not compatible with science probes—yet. So that may limit its use in science courses that require plug-in data collection tools. Christina Tynan-Wood has been covering technology since the dawn of the Internet and currently writes the Family Tech column for Family Circle. You can find more advice about buying and using technology at GeekGirlfriends.com. Follow her on Twittter!