Desmos Devices for Exams: a Brief Onboarding for Instructors

Preparing the Exam for Students to Use a Device

Desmos has some capabilities to be aware of when giving students a paper exam.

Desmos will report approximate decimal values for the coordinates of intersection points, intercepts, roots, and extrema upon tapping/clicking the point. If the intent of an exam question is the assess students’ algebra skills, consider asking for an exact value (not a decimal approximation) or asking for a demonstration of how to calculate the value.
Desmos can plot the solutions \(x\) to an equation implicitly defined in terms of \(x.\) This can be used to answer any solve this equation for \(x\) question up to a few decimals of accuracy just by entering the equation into Desmos. Consider asking for an exact value (not decimal approximation) or asking to demonstrate how to calculate the value. Using a variable other than \(x\) may prevent students from discovering this feature.
There are a few “example” plots available to students under the Saved Graphs menu (folder icon, top-left corner). However these would be helpful for only the most basic questions.

Preparing the Students to Use a Device on an Exam

Note that the devices don’t have keyboards, only touch interfaces, so when demonstrating how to use the app on the classroom computer try to use only the mouse to simulate screen touches. Desmos offers a pop-up on-screen keyboard in the web app for this purpose.
Do a class demonstration of Desmos’ features. E.g. panning and pinching the viewport, manually changing the viewport (wrench icon), storing values to variables, defining and evaluating functions, built-in functions (e.g. sqrt and log), adding a table, adding a column for function outputs to a table, adding a regression formula from a table setting the regression formula as the formula for a function, etc. Show the students the documentation for Desmos under the (?) help icon.

It may be a good idea to become familiar with Desmos’s advanced features to (1) understand exactly what Desmos is capable of, or to (2) prepare for a precocious student interested in learning more. In particular, sliders and movable points, dynamic labels, custom colors, lists, the “for” keyword, recursion and advanced recursion, actions, and complex numbers.
Urge students to use Desmos installed on their own device in class instead of a TI-83/84+ graphing calculator since that’s what they’ll use on the exams.
The online homework on regression for power, logarithmic, and exponential functions was calibrated on a TI-83/84+ calculator. The short story is that, by default, Desmos computes these regression formulas differently than a TI. Make sure students know that they can tick ✓ the Log Mode setting under these regression formulae in Desmos to make it match the online homework’s expected answers.

The long story: a TI-83/84+ calculator, with its limited computational power, will “linearize” the data so it can perform linear regression, then transform the linear formula back into the power/log/exp model as appropriate. For example, when running PwrReg on some data \({\bigl\{(x_i, y_i)\bigr\},}\) the TI calculator will first transform the data to \({\bigl\{\bigl(\ln(x_i), \ln(y_i)\bigr)\bigr\},}\) perform linear regression to compute the linear model \(\ln(y_1) \sim {a + b\ln(x_1),}\) then exponentiate both sides before displaying the power model \(y_1 \sim {\mathrm{e}^a\ln(x_1^b)}\) with parameters \(\mathrm{e}^a\) and \(b.\) Similarly for LogReg and ExpReg the calculator will perform linear regression on the transformed data \({\bigl\{\bigl(\ln(x_i), y_i\bigr)\bigr\}}\) and \({\bigl\{\bigl(x_i, \ln(y_i)\bigr)\bigr\}}\) respectively.

Preparing the Device for Students to Use on an Exam

Before the exam period, lay out all the phones face up on a large surface with gaps between them. Going row-by-row with assembly line efficiency, (1) power them on, (2) unlock them, (3) start Desmos on each, (4) press the App Switcher (square) on each, (5) pin Desmos on each, (6) press Got it to dismiss the “App is pinned” notification on each, and (7) relock them. Takes 5–10 minutes. This goes more quickly if Desmos is still open on the device from when it was last shutdown.
In the classroom, before handing out the exams themselves, proceed to hand out a device to each student, unlocking first it to double-check Desmos is pinned.
Consider casually mentioning that they’re Walmart TracFones to dissuade theft.
Announce and write on the board: Hand in your exam and device together and be sure to collect the devices in this fashion to ensure no student who hands in an exam has not also handed in their device.
Upon being handed the device, there’s no need to clear/close Desmos; just turn it off by pressing and holding the power-button and volume-up-button simultaneously. This way Desmos will remain open but cleared when the phone is restarted.
Note, the devices don’t stack well, so they should be laid into a box on their edge, stacked like records in a milk crate. When doing this, be sure to stack them with the power button pointed upward so the device doesn’t accidentally power on.