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OK, class — pull out your smartphones, please.

It's not the conventional way to begin math class, but neither is Sara Van Der Werf's calculator alternative at Minneapolis South High School. Instead of banking on clunky and pricey TI-84 calculators, her kids use a free graphing calculator app on their mobile devices.

Students pinch to zoom into graphs of algebraic equations on the Desmos app. They touch curves to show maximums and minimums. Best of all, they can crunch equations, whether in class or at home, without having to buy a costly calculating tool.

"I saw it almost immediately as kind of an equity game-changer for students everywhere," said Van Der Werf, who teaches advanced algebra.

Teachers across the Minneapolis district, at South and Southwest high schools, and at Ramsey and at Sanford middle schools, are using the app, Van Der Werf said.

Desmos, a San Francisco-based company, launched its online calculator in 2012 and its app in 2014. Both are used worldwide, said Christopher Danielson, a member of Desmos' teaching faculty. He lives in St. Paul and is a math instructor on leave from Normandale Community College.

Van Der Werf has been trying Desmos on phones for about a year, though she also circulates regular graphing calculators through her classroom.

The expensive tools necessary for higher-level math work can put some students at a disadvantage, she said. The graphing calculator used in Van Der Werf's classes is the TI-84, manufactured by Texas Instruments. Depending on the model, they can cost more than $100.

About half of South's students qualify for free or reduced lunches, according to Minnesota Department of Education data, so forking over cash for a math tool could be an economic hardship. South High Principal Ray Aponte said that since he became principal in 2014, students have never been required to buy a costly calculator.

"We cannot have programming where students may not be able to access something based on, you know, they just don't have a piece of equipment," he said.

Most of these students do have access to a smartphone, Van Der Werf said.

Text, Snapchat and calculus

Using the online calculator, students can plug in equations and watch as line graphs and parabolas plop onto x- and y-axes. Hovering with a mouse shows intercepts. A slide button makes the graphs bend with changing variables. Using the app, they can tap a curve to reveal maximums and minimums.

Van Der Werf, who is president of the Minnesota Council of Teachers of Mathematics, has a classroom calculator museum to showcase her collection of computational devices. She said she loves that students don't need Wi-Fi to use the app at home. She can give out math homework that asks students to craft graphs and tables, and check their work.

Plus, it's intuitive, she said, and after students get used to it, they often favor Desmos.

The downside? Kids can't use the app or online calculator when they take their ACT or MCA tests. Danielson said Desmos is working to change that.

He acknowledges that critics of technological math education say that the more devices students use, the less they'll learn about crunching numbers themselves. His rebuttal: Kids can use calculators in ways that are meaningful.

At South, Van Der Werf said she and probably two other teachers are "really going hard with the cellphones" in math class.

"As I look at it and forecast, I could see in five to 10 years, this will be the norm," she said.

Beena Raghavendran • 612-673-4569