On a dark gray blackboard, many mathematical formulas are written in chalkOn a dark gray blackboard, many mathematical formulas are written in chalk
Lecture with formulas on the blackboard in front of studentsLecture with formulas on the blackboard in front of students
Widescreen wallpaper girl looking dreamily into the distanceWidescreen wallpaper girl looking dreamily into the distance
The formula of water is written in drops on a planeThe formula of water is written in drops on a plane