You can find the value 2*y0, when the derivative of 1/x cut the Y exe, by making similar triangles. If the slope reflects a change from y0 to 0 when X changes from x0 to 2*x0 , so it should change from y0 to 2*y0 when X changes from x0 to 0. It's easiest to see in a graph.The notation f' is from Lagrange, not Newton. I love MITOCW. Thanks!####[['MITOCW', 'Course_General_Feedback', 'POS']]