As you know, when a course ends, students start to forget the material they have learned. one model (called the ebbinghaus model) assumes that the rate at which a student forgets material is proportional to the difference between the material currently remembered and some positive constant, a. a. let y=f(t)be the fraction of the original material remembered t weeks after the course has ended. set up a differential equation for y, using k as any constant of proportionality you may need (let k> 0). your equation will contain two constants; the constant a (also positive) is less than y for all t.