Classroom Desmos Devices

Preparing the Exam for Students to Use a Device

Desmos has some capabilities to be aware of when assessing students.

  1. Desmos will report approximate decimal values for the coordinates of intersection points, vertices, roots, etc upon tapping the screen. This can be used to answer any solve this equation for \(x\) up to a few decimals of accuracy just by entering the equation into Desmos. To subvert this, consider asking students for exact values, or asking students to demonstrate how to calculate the value.
  2. There are a few “examples” available to students under the hamburger menu . However these would only be helpful for only the most basic questions.

Preparing the Students to Use a Device on an Exam

  1. Show students all the available documentation:

    Ideally use class time to do a demonstration of the features of the app you’ll be using. E.g. panning and pinching the viewport, manually changing the viewport (wrench icon), defining function and function evaluation, built-in functions (e.g. sqrt), storing values to variables, adding a table, adding a column for function outputs to a table, adding a regression formula from a table and setting that as the formula for a function, sliders, colors, etc.

  2. Urge students to use the specified app installed on their own device instead of a TI-83/84+ graphing calculator in class, since that’s what they’ll use on the exams.
  3. 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; avoid the keyboard. Desmos offers a pop-up on-screen keyboard in the web app for this purpose.

  4. The online homework on regression for power, logarithmic, and exponential functions was calibrated on a TI-83/84+ calculator. By default Desmos computes these regression formulas differently. The short answer: make sure students know that, in Desmos, they can tick ✓ the Log Mode setting under these regression formulae to make it match the online homework.

    The long story: TI Calculators, with their limited computational power, will “linearize” the data so it can perform linear regression, the transform the linear formula back into the power/log/exp model as appropriate. For example, when running PwrReg on the data \(\bigl\{(x_i, y_i)\bigr\},\) the TI calculator will transform the data to \(\bigl\{\bigl(\ln(x_i), \ln(y_i)\bigr)\bigr\}\) first, perform linear regression to find the linear model \(\ln(y_1) \sim a + b\ln(x_1),\) and exponentiate both sides before returning 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.

    This also addressees a long-standing question I've had about why TI chose those specific templates for their models. Using Desmos, we are no longer limited by that choice of template.

Preparing the Device for Students to Use on an Exam

  1. Before the exam period, lay out all the phones face up on a large surface with space between them, and 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 quicker if Desmos is still open on the device from when it was last shutdown.
  2. In the classroom, before handing out the exams themselves, proceed to hand out a device to each student, unlocking first it to make sure Desmos is pinned.
  3. Consider casually mentioning they’re cheap Walmart TracFones to dissuade theft.
  4. 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.
  5. 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.

  6. Protip

    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.