readme: https://teacher.desmos.com/activitybuilder/teacherguide/604249659240440d25a27d0c
  https://teacher.desmos.com/activitybuilder/teacherguide/604249a365ecd40d30b4ad18
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  https://teacher.desmos.com/activitybuilder/teacherguide/60424e030d98a80d53856ab2
  https://teacher.desmos.com/activitybuilder/teacherguide/60424e37ed488c0cfbbaab2f