I have been reading Ordinary Differential Equations (Pollard) from Dover. The chapter I am in, is called Problems Leading to Differential Equations of The First Order - Geometric Problems. Problem : Find the family of curves with the property that the area of the region bounded by the x axis , the tangent line drawn at a point P(x,y) of a curve of the family and the projection of the tangent line on the x axis has a constante value A. image hosting png In the solution, they say the equation of the tangent line is y / (x - a) = y' They then solve, for a: a = x - (y/y') Afterwards, they obtain the distance QR = y/y' Therefore they have the area of the triangle. They integrate, bla blabla. Now, when I first looked this, it seemed pretty simple and straighforward. I understood every step. It was an elementary problem. But, today I gave it a second look, and now I just don't agree with the solution. --------------- Well, my question is y = mx + b; but m = y'. so, y = y' x + b. I don't agree with this since y defines the equation of the tangent line BUT y' defines the derivative of THE CURVE. therefore in my viewing, when they, in the solution, reach to QR = y/y', and then integrate they are mixing a fuction and a derivative of a diferent fuction. So, where is my reasoning wrong? Perhaps I should sleep more. ;D Thanks for all the explanations!